Test with 1.21 instead of 1.21.0

I'm assuming this will get us the latest 1.21 release (currently
1.21.13) instead of 1.21.0.

https://go.dev/doc/devel/release#go1.21.minor

Add comments for when to update version based on discussion in
#154

Author: jmr

.github/workflows/go.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       matrix:
         # Test on supported go compiler releases as well as the oldest version we support from go.mod.
-        go: [ '1.21.0', 'oldstable', 'stable' ]
+        go: [ '1.21', 'oldstable', 'stable' ]
     name: Go version ${{ matrix.go }}
     steps:
     - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2

go.mod

@@ -1,5 +1,7 @@
 module github.com/golang/geo
 
+# This declares what language features we use and may be updated freely up to "oldstable".
+# Update .github/workflows/go.yml when bumping this version.
 go 1.21.0
 
 require github.com/google/go-cmp v0.7.0 // indirect