arti hss ctor-migrate does not migrate the restricted discovery clients

Follow-up from !3102 (merged)

By design, arti hss ctor-migrate is a tool that migrates keys from a C Tor keystore to the Arti native one. The migration process does not actually "migrate" the authorized_clients of the C Tor service (the user still has to manually add them to the restricted_discovery section from the arti config). I think this could be a footgun, because users might wrongly assume their old C Tor keys directory is no longer needed once the migration is complete.

cc @hjrgrn