ci: go build: add Go 1.21.x

Author: grokify

.github/workflows/test.yaml

@@ -11,7 +11,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.20.x, 1.13.x]
+        go-version: [1.21.x, 1.13.x]
         platform: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps: