[ci] Disable ubuntu-18.04 jobs

Github has now dropped support for running jobs on Ubuntu 18.04 and
these jobs now sit un-run for a day and then fail.

Hopefully many of these jobs can be resurrected on a newer Ubuntu
version, but as a first step let's get git master back to a state where
all the enabled jobs can be expected to pass.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ba1e041..dca0d3d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -75,21 +75,21 @@
         - 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: '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'
@@ -115,12 +115,12 @@
           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: '3.3'
+        #  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
@@ -146,21 +146,21 @@
         - 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
+        #- 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
         - SWIGLANG: ruby
           VER: '2.4'
         - SWIGLANG: ruby
@@ -177,8 +177,8 @@
           CPPSTD: c++11
         - SWIGLANG: scilab
           VER: '5.5.2'
-        - SWIGLANG: scilab
-          os: ubuntu-18.04 # scilab 6.0
+        #- SWIGLANG: scilab
+        #  os: ubuntu-18.04 # scilab 6.0
         - SWIGLANG: scilab
         - SWIGLANG: tcl
         # c++11 testing
@@ -309,11 +309,11 @@
         # 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
+        #- 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
       # Run all of them, as opposed to aborting when one fails
       fail-fast: false