[ci] Try simply running ubuntu-18.04 jobs on 20.04
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index dca0d3d..9601d1b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -32,288 +32,49 @@
     strategy:
       matrix:
         include:
-        - SWIGLANG: ""
-          CPPFLAGS: "-DDOH_POISON"
-        - SWIGLANG: ""
-          GCC: 7
-        - SWIGLANG: ""
-          GCC: 8
-        - SWIGLANG: ""
-          GCC: 9
-        - SWIGLANG: ""
-          GCC: 10
-        - SWIGLANG: ""
-          GCC: 11
-        - SWIGLANG: ""
-          GCC: 12
-          os: ubuntu-22.04
-        - SWIGLANG: ""
-          compiler: clang
-        - SWIGLANG: csharp
-        # D support can't be enabled because dmd 2.066 fails to build anything
-        # under Ubuntu 18.04 due to its standard library (libphobos2.a) not
-        # being compiled with -FPIC, but system gcc using -fpie by default,
-        # resulting in linking errors for any output. And later versions, such
-        # as 2.086.1, are not supported and result in errors in SWIG test suite.
-        #
-        # - SWIGLANG: d
-        #   VER: '2.066.0'
-        #   os: ubuntu-18.04 # This dlang version doesn't work under 20.04.
-        - SWIGLANG: go
-          VER: '1.6'
-          CSTD: gnu11
-        - SWIGLANG: go
-          VER: '1.8'
-        - SWIGLANG: go
-          VER: '1.12'
-          CSTD: gnu11
-        - SWIGLANG: go
-          VER: '1.17'
-        - SWIGLANG: guile
-        - SWIGLANG: guile
+        - SWIGLANG: javascript
+          ENGINE: node
+          VER: '6'
+          CPPSTD: c++11
+          os: ubuntu-20.04
+        - SWIGLANG: javascript
+          ENGINE: node
+          VER: '8'
+          CPPSTD: c++11
+          os: ubuntu-20.04
+        - SWIGLANG: javascript
+          ENGINE: node
+          VER: '10'
+          CPPSTD: c++11
+          os: ubuntu-20.04
+        - SWIGLANG: python
+          VER: '3.3'
+          os: ubuntu-20.04 # Python < 3.5 not available for 20.04.
+        - SWIGLANG: python
+          VER: '3.4'
+          os: ubuntu-20.04 # Python < 3.5 not available for 20.04.
+        - SWIGLANG: ruby
+          VER: '1.9'
+          os: ubuntu-20.04
+        - SWIGLANG: ruby
+          VER: '2.0'
+          os: ubuntu-20.04
+        - SWIGLANG: ruby
+          VER: '2.1'
+          os: ubuntu-20.04
+        - SWIGLANG: ruby
           VER: '2.2'
-        - SWIGLANG: guile
-          VER: '3.0'
-        - SWIGLANG: java
-        #- SWIGLANG: javascript
-        #  ENGINE: node
-        #  VER: '6'
-        #  CPPSTD: c++11
-        #  os: ubuntu-18.04
-        #- SWIGLANG: javascript
-        #  ENGINE: node
-        #  VER: '8'
-        #  CPPSTD: c++11
-        #  os: ubuntu-18.04
-        #- SWIGLANG: javascript
-        #  ENGINE: node
-        #  VER: '10'
-        #  CPPSTD: c++11
-        #  os: ubuntu-18.04
-        - SWIGLANG: javascript
-          ENGINE: node
-          VER: '12'
-          CPPSTD: c++11
-        - SWIGLANG: javascript
-          ENGINE: node
-          VER: '18'
-          CPPSTD: c++14
-        - SWIGLANG: javascript
-          ENGINE: jsc
-          VER: '4.0'
-        - SWIGLANG: lua
-        - SWIGLANG: lua
-          VER: '5.3'
-        - SWIGLANG: octave
-          CPPSTD: c++11
-        - SWIGLANG: perl5
-        - SWIGLANG: php
-          VER: '8.0'
-        - SWIGLANG: php
-          VER: '8.1'
-        - SWIGLANG: php
-          VER: '8.2'
-        - SWIGLANG: python
-          PY2: 2
-        #- SWIGLANG: python
-        #  VER: '3.3'
-        #  os: ubuntu-18.04 # Python < 3.5 not available for 20.04.
-        #- SWIGLANG: python
-        #  VER: '3.4'
-        #  os: ubuntu-18.04 # Python < 3.5 not available for 20.04.
-        - SWIGLANG: python
-          VER: '3.5'
-        - SWIGLANG: python
-          VER: '3.6'
-        - SWIGLANG: python
-          VER: '3.7'
-        - SWIGLANG: python
-          VER: '3.8'
-        - SWIGLANG: python
-          VER: '3.9'
-        - SWIGLANG: python
-          VER: '3.10'
-        - SWIGLANG: python
-          VER: '3.11'
-        - SWIGLANG: python
-          PY2: 2
-          SWIG_FEATURES: -builtin
-        - SWIGLANG: python
-          PY2: 2
-          SWIG_FEATURES: -builtin -O
-        - SWIGLANG: python
-          SWIG_FEATURES: -builtin
-        - SWIGLANG: python
-          SWIG_FEATURES: -builtin -O
-        - SWIGLANG: r
-        #- SWIGLANG: ruby
-        #  VER: '1.9'
-        #  os: ubuntu-18.04
-        #- SWIGLANG: ruby
-        #  VER: '2.0'
-        #  os: ubuntu-18.04
-        #- SWIGLANG: ruby
-        #  VER: '2.1'
-        #  os: ubuntu-18.04
-        #- SWIGLANG: ruby
-        #  VER: '2.2'
-        #  os: ubuntu-18.04
-        #- SWIGLANG: ruby
-        #  VER: '2.3'
-        #  os: ubuntu-18.04
+          os: ubuntu-20.04
         - SWIGLANG: ruby
-          VER: '2.4'
-        - SWIGLANG: ruby
-          VER: '2.5'
-        - SWIGLANG: ruby
-          VER: '2.6'
-        - SWIGLANG: ruby
-          VER: '2.7'
-        - SWIGLANG: ruby
-          VER: '3.0'
-          CPPSTD: c++11
-        - SWIGLANG: ruby
-          VER: '3.1'
-          CPPSTD: c++11
+          VER: '2.3'
+          os: ubuntu-20.04
         - SWIGLANG: scilab
-          VER: '5.5.2'
-        #- SWIGLANG: scilab
-        #  os: ubuntu-18.04 # scilab 6.0
-        - SWIGLANG: scilab
-        - SWIGLANG: tcl
-        # c++11 testing
-        - SWIGLANG: csharp
-          CPPSTD: c++11
-        - SWIGLANG: go
-          VER: '1.17'
-          CPPSTD: c++11
-          CSTD: gnu11
-        - SWIGLANG: guile
-          CPPSTD: c++11
-        - SWIGLANG: java
-          CPPSTD: c++11
-        - SWIGLANG: javascript
-          ENGINE: jsc
-          VER: '4.1'
-          os: ubuntu-22.04
-          CPPSTD: c++11
-        - SWIGLANG: javascript
-          ENGINE: node
-          VER: '14'
-          CPPSTD: c++11
-        - SWIGLANG: lua
-          CPPSTD: c++11
-        - SWIGLANG: perl5
-          CPPSTD: c++11
-        - SWIGLANG: php
-          CPPSTD: c++11
-          CSTD: gnu11
-        - SWIGLANG: python
-          CPPSTD: c++11
-        - SWIGLANG: r
-          CPPSTD: c++11
-        - SWIGLANG: ruby
-          CPPSTD: c++11
-        - SWIGLANG: scilab
-          CPPSTD: c++11
-        - SWIGLANG: tcl
-          CPPSTD: c++11
-        # c++14 testing
-        - SWIGLANG: csharp
-          CPPSTD: c++14
-        - SWIGLANG: go
-          VER: '1.17'
-          CPPSTD: c++14
-          CSTD: gnu11
-        - SWIGLANG: guile
-          CPPSTD: c++14
-        - SWIGLANG: java
-          CPPSTD: c++14
-        - SWIGLANG: javascript
-          ENGINE: node
-          VER: '16'
-          CPPSTD: c++14
-        - SWIGLANG: lua
-          CPPSTD: c++14
-        - SWIGLANG: octave
-          CPPSTD: c++14
-        - SWIGLANG: perl5
-          CPPSTD: c++14
-        - SWIGLANG: php
-          CPPSTD: c++14
-          CSTD: gnu11
-        - SWIGLANG: python
-          CPPSTD: c++14
-        - SWIGLANG: r
-          CPPSTD: c++14
-        - SWIGLANG: ruby
-          CPPSTD: c++14
-        - SWIGLANG: scilab
-          CPPSTD: c++14
-        - SWIGLANG: tcl
-          CPPSTD: c++14
-        # c++17 testing (using gcc11)
-        - SWIGLANG: csharp
+          os: ubuntu-20.04 # scilab 6.0
+        - SWIGLANG: ocaml
           CPPSTD: c++17
           GCC: 11
-        - SWIGLANG: go
-          VER: '1.17'
-          CPPSTD: c++17
-          GCC: 11
-          CSTD: gnu17
-        - SWIGLANG: guile
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: java
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: javascript
-          ENGINE: node
-          VER: '18'
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: lua
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: octave
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: perl5
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: php
-          CPPSTD: c++17
-          CSTD: gnu17
-          GCC: 11
-        - SWIGLANG: python
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: r
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: ruby
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: scilab
-          CPPSTD: c++17
-          GCC: 11
-        - SWIGLANG: tcl
-          CPPSTD: c++17
-          GCC: 11
-        # c++20 testing (using gcc12)
-        # ubuntu-22.04 is currently experimental on Github Actions, so limit to just one language for now
-        - SWIGLANG: python
-          CPPSTD: c++20
-          GCC: 12
-          os: ubuntu-22.04
-        # Experimental languages (these are allowed to fail)
-        - SWIGLANG: mzscheme
           continue-on-error: true
-        #- SWIGLANG: ocaml
-        #  CPPSTD: c++17
-        #  GCC: 11
-        #  continue-on-error: true
-        #  os: ubuntu-18.04 # ocaml-4.08 in ubuntu-20.04 not yet working
+          os: ubuntu-20.04 # ocaml-4.08 in ubuntu-20.04 not yet working
       # Run all of them, as opposed to aborting when one fails
       fail-fast: false