50.alpha
* Fixed bug where we'd lose track of the user session failure counter
* Added GNOME OS CI
* Special sessions (like GDM, initial-setup) are now loaded from .desktop files
  rather than being hard-coded into GDM
* Entirely removed X11 support, except for the ability to launch other desktops'
  X11 sessions with a per-user X server. This means that features like XDCMP and
  system-wide Xserver are gone, and that GDM's own sessions will always run with
  Wayland.
* Consequently, we removed the ability to compile GDM without Wayland support
* Introduced a new `gnome-headless-session@<username>.service`, which makes it
  easier to start a headless graphical session (i.e. for RDP purposes)
* Merged display and session registration into one step, because the previous
  approach didn't work right for Wayland. This allowed us to remove workarounds
  that caused bugs
* Reworked wtmp/utmp/btmp fields to contain more useful values, especially on
  Wayland and with headless RDP sessions
* Dropped final usages of the `gdm`/`gdm3` user, so distros no longer need
  to create it for us (GDM now entirely relies on dynamically allocated users)
* Adapted to changes in gnome-shell's systemd unit files, and dropped hard-coded
  gnome-shell session modes. It's now easier for custom/kiosk sessions to use
  custom gnome-shell session modes
* Added support for Linux 6.18's new `boot_display` sysfs attribute, which
  replaces the `boot_vga` attribute. These attributes allow GDM to identify
  the primary GPU, but modern AMD cards cannot use `boot_vga` because they
  no longer have VGA support!
* Fixed bug where we failed to propagate some environment variables into
  the greeter's Pipewire instance