ci: update for go1.21

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b94f980..24e6236 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,7 +12,7 @@
       matrix:
         # Do not move this line; it is referred to by README.md.
         # Versions of Go that are explicitly supported by Gonum.
-        go-version: [1.20.x, 1.19.x]
+        go-version: [1.21.x, 1.20.x]
         platform: [ubuntu-latest, macos-latest]
         force-goarch: ["", "386"]
         tags: 

.github/workflows/codecov.yml

diff --git a/.github/workflows/codecov.yml b/.github/workflows/codecov.yml
index dc49bb6..b212bbd 100644
--- a/.github/workflows/codecov.yml
+++ b/.github/workflows/codecov.yml
@@ -13,7 +13,7 @@
     if: github.event.pull_request.merged == true
     strategy:
       matrix:
-        go-version: [1.20.x]
+        go-version: [1.21.x]
         platform: [ubuntu-latest]
         tags: 
           - ""

go.mod

diff --git a/go.mod b/go.mod
index de9f582..5c20a2f 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module gonum.org/v1/gonum
 
-go 1.18
+go 1.20
 
 require (
 	github.com/goccmack/gocc v0.0.0-20230228185258-2292f9e40198