Format with Go 1.19 formatter (#3277)

* Format with Go 1.19 formatter

This allows the GoDoc to take advantage of new markup syntax introduced in Go 1.19. This does not require that our minimum supported version be bumped to Go 1.19 since the pkgsite renders our godoc regardless of supported Go version.

* Add Format check

.github/workflows/gin.yml

@@ -68,6 +68,10 @@ jobs:
         uses: codecov/codecov-action@v3
         with:
           flags: ${{ matrix.os }},go-${{ matrix.go }},${{ matrix.test-tags }}
+
+      - name: Format
+        if: matrix.go-version == '1.19.x'
+        run: diff -u <(echo -n) <(gofmt -d .)
   notification-gitter:
     needs: test
     runs-on: ubuntu-latest