Firestore/CHANGELOG.md: add release notes entry

Author: dconeybe

Firestore/CHANGELOG.md

@@ -2,6 +2,9 @@
 - [fixed] Fixed an issue where Firestore's binary SwiftPM distribution would
   not link properly when building a target for testing. This issue affected
   Xcode 15 Beta 5 and later (#11656).
+- [fixed] Downgrade the CocoaPods grpc dependency back to 1.44.0 (from 1.50.1)
+  to fix a crash on iOS 12 devices that was introduced in the Firebase Apple SDK
+  10.10.0 when the grpc dependency was upgraded (#11509).
 
 # 10.15.0
 - [feature] Add the option to allow the SDK to create cache indexes automatically to