The previous value (`default.target`) meant that redshift got started when a user session began, at which point we can't expect a graphic session to be up and running (eg: Xorg or Wayland may have not even been invoked yet). Setting `WantedBy=graphical-session.target` ensures that, when the service is enable, it'll only get run when starting a graphical session (again, either Xorg or Wayland, whatever DE's may be used).