Update CHANGELOG for Firestore v1.13.0 (#5491)

Author: wilhuff

Firestore/CHANGELOG.md

@@ -1,7 +1,10 @@
 # Unreleased
+
+# v1.13.0
 - [changed] Firestore now limits the number of concurrent document lookups it
   will perform when resolving inconsistencies in the local cache
   (https://github.com/firebase/firebase-js-sdk/issues/2683).
+- [changed] Upgraded gRPC-C++ to 1.28.0 (#4994).
 - [fixed] Firestore will now send Auth credentials to the Firestore Emulator
   (#5072).