Add deprecation warning for the snap_daemon system username #5448

What needs to get done

Snapcraft should add a deprecation notice if the snap_daemon system username is used.

system-usernames:
  snap_daemon: shared

Warning: The system username 'snap_daemon' is deprecated. See https://snapcraft.io/docs/system-usernames.

Why it needs to get done

Because snapd has deprecated the username. See https://snapcraft.io/docs/system-usernames.