Add a changelog entry on Protobuf removal. (#4266)

* Add a changelog entry on `Protobuf` removal.

Removed in #4189.

Author: var-const

Firestore/CHANGELOG.md

@@ -1,4 +1,7 @@
 # Unreleased
+- [changed] Removed the Firestore's dependency on `Protobuf`. If you're manually
+  tracking dependencies, you may be able to remove it from your build (note,
+  however, that other Firebase components may still require it).
 
 # v1.7.0
 - [feature] Added `whereField(_:in:)` and `whereField(_:arrayContainsAny:)` query