chore(firebase-perf): Move Changelog entry to Unreleased section

Author: exaby73

firebase-perf/CHANGELOG.md

@@ -1,9 +1,9 @@
 # Unreleased
 
-
-# 21.0.2
 * [fixed] Fixed a performance issue with shared preferences
   calling `.apply()` every time a value is read from remote config (#6407)
+
+# 21.0.2
 * [fixed] Fixed `IllegalStateException` that happened when starting a trace
   before Firebase initializes.
 * [changed] Updated protobuf dependency to `3.25.5` to fix