go.yml: update to 1.21 (#58)

This matches the go.mod files of the two modules.

Author: jba

.github/workflows/go.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: '1.20'
+        go-version: '1.21'
 
     - name: Test
       run: |