Fix to-device messages not being sent to MSC3202-enabled appservices (#13235)

The field name was simply incorrect, leading to errors.

changelog.d/13235.bugfix

@@ -0,0 +1 @@
+Fix MSC3202-enabled appservices not receiving to-device messages, preventing messages from being decrypted.

synapse/appservice/scheduler.py

@@ -331,8 +331,9 @@ class _ServiceQueuer:
             )
 
         # Add recipients of to-device messages.
-        # device_message["user_id"] is the ID of the recipient.
-        users.update(device_message["user_id"] for device_message in to_device_messages)
+        users.update(
+            device_message["to_user_id"] for device_message in to_device_messages
+        )
 
         # Compute and return the counts / fallback key usage states
         otk_counts = await self._store.count_bulk_e2e_one_time_keys_for_as(users)