Make cleaning up pushers depend on the device_id instead of the token_id (#15280)

This makes it so that we rely on the `device_id` to delete pushers on logout,
instead of relying on the `access_token_id`. This ensures we're not removing
pushers on token refresh, and prepares for a world without access token IDs
(also known as the OIDC).

This actually runs the `set_device_id_for_pushers` background update, which
was forgotten in #13831.

Note that for backwards compatibility it still deletes pushers based on the
`access_token` until the background update finishes.

synapse/handlers/register.py

@@ -1013,11 +1013,11 @@ class RegistrationHandler:
             user_tuple = await self.store.get_user_by_access_token(token)
             # The token better still exist.
             assert user_tuple
-            token_id = user_tuple.token_id
+            device_id = user_tuple.device_id
 
             await self.pusher_pool.add_or_update_pusher(
                 user_id=user_id,
-                access_token=token_id,
+                device_id=device_id,
                 kind="email",
                 app_id="m.email",
                 app_display_name="Email Notifications",