Remove instanceID dependency in Messaging (#7942)

Author: charlotteliang

FirebaseMessaging/CHANGELOG.md

@@ -1,3 +1,6 @@
+# unreleased
+- [changed] Remove Instance ID dependency from Messaging. This is a breaking change for Firebase Messaging users who use Instance ID to manage registration tokens without explicitly specifying Instance ID in their podfile or Swift Package Manager. For a short-term resolution, those Messaging users can update their dependencies to explicitly specify Instance ID. Longer term, all users should migrate to Messaging's token APIs following the migration guide: https://firebase.google.com/docs/projects/manage-installations. (#7836)
+
 # 2021-04 -- v7.11.0
 - [changed] Refactor Messaging to internally not depending on InstanceID, but can co-exist. Will remove InstanceID dependency in the next Firebase breaking change. (#7814)