Autostart using systemd instead of gnome-session

[ryonakano]

Problem

Since newer version of gnome-session on Wayland, it no longer manages session services: https://discourse.gnome.org/t/autostart-files-with-x-gnome-autostart-phase-entry-in-gnome-49/31620/9

Proposal

Migrate autostart.desktop to systemd service on OS 9, which would be based on Ubuntu 26.04 LTS with GNOME 50.

Prior Art (Optional)

elementary/settings-daemon#218

[ryonakano]

Oh, I realized just now that autostart.desktop of Onboarding doesn't use X-GNOME-Autostart-Phase. Using XDG autostart here should be an expected usage because it's a GUI application not a session service, so I'll close this issue as invalid.