CI: Add CI check for what's new link (#70046)

* Add whatsnewchecker

* Add whatsnewchecker_test

* Small fixes

* Add step in CI

* Fix lint

* Fix starlark

* t.Cleanup instead of separate func

* Skip check for test tags

pkg/build/cmd/main.go

@@ -82,6 +82,11 @@ func main() {
 				&buildIDFlag,
 			},
 		},
+		{
+			Name:   "whatsnew-checker",
+			Usage:  "Checks whatsNewUrl in package.json for differences between the tag and the docs version",
+			Action: WhatsNewChecker,
+		},
 		{
 			Name:   "build-docker",
 			Usage:  "Build Grafana Docker images",