add changelog

Author: David Motsonashvili

firebase-ai/CHANGELOG.md

@@ -4,6 +4,7 @@
   have not been granted by the user.
 - [feature] Added helper functions to `LiveSession` to allow developers to track the status of the
   audio session and the underlying websocket connection.
+- [changed] Added new values to `HarmCategory` (#7324)
 
 # 17.2.0