refactor(cloud_firestore): use firebase.google.com link for `homepa…

…ge` in `pubspec.yaml` (#8724)

Co-authored-by: Russell Wheatley <russellwheatley85@gmail.com>

packages/cloud_firestore/cloud_firestore/pubspec.yaml

@@ -2,7 +2,7 @@ name: cloud_firestore
 description:
   Flutter plugin for Cloud Firestore, a cloud-hosted, noSQL database with
   live synchronization and offline support on Android and iOS.
-homepage: https://firebase.flutter.dev/docs/firestore/overview
+homepage: https://firebase.google.com/docs/firestore
 repository: https://github.com/firebase/flutterfire/tree/master/packages/cloud_firestore/cloud_firestore
 version: 3.1.17