Update gdm-tails.json for Bullseye

This is about config/gdm-tails.json from greeter.git, which might move to tails.git when #16912 (closed) is done.

It looks like this file is a kind of hybrid between classic.json (meant for a regular GNOME Classic user session) and the gdm mode in js/ui/sessionMode.js (from GNOME Shell’s source tree).

We should make sure that the file is updated if one of the files it’s based on is updated in Debian. And then we should create a similar ticket targeting Tails 6.0.

Further relevant findings from #12551 (closed):

• gdm-tails.json was created in 2016, for Tails based on Jessie I assume. Jessie had gnome-shell-extensions 3.14.2, which includes data/classic.json.in. That file wasn’t updated since then, so I suspect all the differences are on purpose.
• Our panelStyle is loginScreen while GNOME’s is login-screen. login-screen is defined in gnome-shell’s CSS, loginScreen does not seem to be defined anywhere, so it has probably no effect. We could either change it to actually use the login-screen style or delete the panel style to keep using the default panel style.

Related issues

• Related to #12551 (closed)

Original created by @segfault on 16951 (Redmine)