update changelog in core for removing instanceID (#7985)

Author: charlotteliang

FirebaseCore/CHANGELOG.md

@@ -1,5 +1,7 @@
 # Firebase 8.0.0
 - [changed] Migrate `transform:` callsites and introduce breaking version of GDT (9.0). (#7899)
+- [removed] The deprecated Firebase InstanceID has been removed. Use Firebase Installations to manage
+  app instance and use Firebase Messaging to manage FCM registration token instead. (#7970)
 - [removed] Build warnings will no longer be generated to warn about missing capabilities resulting
   from not including FirebaseAnalytics in the app. See the Firebase docs instead. (#7487)
 - [removed] The `Firebase/AdMob` CocoaPods subspec has been removed. Use the `Google-Mobile-Ads-SDK`