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/rest/admin/users.py

@@ -425,7 +425,6 @@ class UserRestServletV2(RestServlet):
                     ):
                         await self.pusher_pool.add_or_update_pusher(
                             user_id=user_id,
-                            access_token=None,
                             kind="email",
                             app_id="m.email",
                             app_display_name="Email Notifications",