Core M66 changelog update (#5013)

* Core M66 changelog update

* Core M65 notes corrected.

Author: maksymmalyhin

FirebaseCore/CHANGELOG.md

@@ -1,4 +1,9 @@
-# Unreleased
+# v6.6.4 -- M66
+- [changed] Added an Apple platform flag (ios/macos/watchos/etc.) to `firebaseUserAgent`.
+  The information will be used to support product decisions related to Apple platforms,
+  e.g. prioritizing watchOS support, etc. (#4939)
+
+# v6.6.3 -- M65
 - [fixed] Fix Zip Builder module map generation that could cause linker missing
   symbol errors in the 6.14.0 through 6.16.0 binary release distributions. (#4819)