Acknowledge user_joined_room/user_left_room in the known-event path

Every membership change logged as an unknown event kind. They carry
nothing the dispatch needs, so listing them as known quiets the log
without changing behavior.

adapter.py

@@ -1480,6 +1480,8 @@ class ChattoAdapter(BasePlatformAdapter):
             "notification_created",
             "new_direct_message_notification",
             "message_retracted",
+            "user_joined_room",
+            "user_left_room",
         ):
             logger.debug(
                 "Chatto: '%s' event received. Not yet implemented or not needed.",