Only send out device list updates for our own users (#12465)

Broke in #12365

synapse/storage/databases/main/devices.py

@@ -1703,7 +1703,9 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
                     next(stream_id_iterator),
                     user_id,
                     device_id,
-                    False,
+                    not self.hs.is_mine_id(
+                        user_id
+                    ),  # We only need to send out update for *our* users
                     now,
                     encoded_context if whitelisted_homeserver(destination) else "{}",
                 )