Update Auth changelog (#6935)

Author: morganchen12

FirebaseAuth/CHANGELOG.md

@@ -1,3 +1,6 @@
+# 7.1.0
+- [fixed] Fixed a crash on iOS 14 when multiple app delegate completion methods were called (#6863).
+
 # 7.0.0
 - [removed] Remove deprecated APIs `dataForKey`,`fetchProvidersForEmail:completion`, `signInAndRetrieveDataWithCredential:completion`, `reauthenticateAndRetrieveDataWithCredential:completion`, `linkAndRetrieveDataWithCredential:completion`. (#6607)
 - [added] Add support for the auth emulator. (#6624)