Update for M84 release. (#6998)

Author: ryanwilson

FirebaseCore/CHANGELOG.md

@@ -1,5 +1,8 @@
 # Firebase 7.2.0
 - [fixed] Reduced `FirebaseApp.configure()` and `+[FIRApp registerInternalLibrary:withName:]` impact on app launch time. (#6902)
+- [added] Added arm64 simulator support to support new Apple silicon based Macs.
+- [changed] Due to the new arm64 simulator support, Xcode 12 is now required for any binary
+  products (Analytics, Performance, zip file distribution).
 
 # Firebase 7.0.0
 - [changed] Update minimum iOS version to iOS 10 except for Analytics which is now iOS 9. (#4847)