Update go.yml

Use Go 1.21 in Github actions.

.github/workflows/go.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: '1.20'
+        go-version: '1.21'
 
     - name: Build
       run: go build -v ./...