Release note for FNextPushId fix (#9192)

Author: paulb777

FirebaseDatabase/CHANGELOG.md

@@ -1,5 +1,6 @@
 # v8.11.0
 - [fixed] Race condition crash in FUtilities.m. (#9096)
+- [fixed] FNextPushId 'successor' crash. (#8790)
 
 # v8.10.0
 - [fixed] Fixed URL handling bug when path is a substring of host. (#8874)