updage changelog (#7540)

FirebaseABTesting/CHANGELOG.md

@@ -1,4 +1,4 @@
-# unreleased
+# v7.7.0
 - [added] Added community support for watchOS. (#7481)
 
 # v7.0.0

FirebaseMessaging/CHANGELOG.md

@@ -1,4 +1,4 @@
-# unreleased
+# 2021-02 -- v7.7.0
 - [fixed] Fixed an issue that when checking storage size before writing to the disk, the client was checking the document folder that is no longer used. (#7480)
 
 # 2021-02 -- v7.6.0

FirebaseRemoteConfig/CHANGELOG.md

@@ -1,4 +1,4 @@
-# unreleased
+# v7.7.0
 - [added] Added community support for watchOS. (#7481)
 
 # v7.6.0