Bug#1107944: gdm3: use upstream 'gdm' for system user name instead of 'Debian-gdm'

[Jeremy Bícha]

Source: gdm3
Version: 48.0-1

According to debian/changelog, Debian began using the custom username
Debian-gdm for the 'gdm' system user in gdm 2.30.5-7 in 2011

  * gdm3.init: chown the configuration to Debian-gdm, to avoid umask
    issues. Closes: #603510.

This means that tools to configure GDM (like the GDM Settings app
which was briefly in Debian) probably don't work in Debian. Guides to
configure GDM probably won't work. See https://bugs.debian.org/1107934

We probably need to add a postinst script to try to move user
configuration over. Technically, any system that had a dconf profile
to modify gdm already had their modifications no longer applied when
updating from bookworm to trixie.

I am not currently working on this bug, but I'm filing this as a
wishlist bug and to provide some background.

Thank you,
Jeremy Bícha