Add a changelog entry for Firestore platform logging (#6603)

Author: var-const

Firestore/CHANGELOG.md

@@ -4,6 +4,7 @@
 - [changed] Internal improvements for future C++ and Unity support. Includes a
   breaking change for the Firestore C++ Alpha SDK, but does not affect
   Objective-C or Swift users.
+- [changed] Added new internal HTTP headers to the gRPC connection.
 
 # v1.18.0
 - [feature] Added `whereField(_:notIn:)` and `whereField(_:isNotEqualTo:)` query