refactor(firebase_messaging): use firebase.google.com link for `hom…

…epage` in `pubspec.yaml` (#8735)

packages/firebase_messaging/firebase_messaging/pubspec.yaml

@@ -1,7 +1,7 @@
 name: firebase_messaging
 description: Flutter plugin for Firebase Cloud Messaging, a cross-platform
   messaging solution that lets you reliably deliver messages on Android and iOS.
-homepage: https://firebase.flutter.dev/docs/messaging/overview
+homepage: https://firebase.google.com/docs/cloud-messaging
 repository: https://github.com/FirebaseExtended/flutterfire/tree/master/packages/firebase_messaging/firebase_messaging
 version: 11.4.0