remove 'xserver' prefix from displayManager.sessionPackages

[atagen]

Hello, and thank you for this repo!

Without changing anything beyond a nix flake update, today I got this while trying to rebuild:

       error: The option `services.xserver.displayManager.sessionPackages' does not exist. Definition values:
       - In `/nix/store/z0pdqcm1ywkha41a95khs6qa3bwr3yki-source/nixos/cosmic/module.nix':
           {
             _type = "if";
             condition = true;
             content = [
               <derivation cosmic-session-0-unstable-2024-04-08>
           ...

per this discussion and commit it appears the correct syntax is now without xserver, much as the cosmic module itself uses desktopManager/displayManager without xserver prefix.

I assume most people using this will be tracking unstable, as I am - unfortunately I'm not aware of a way to satisfy both stable and unstable users right now.

[atagen]

nvm, just saw in another issue that this repo only works with unstable, so should be fine

[lilyinstarlight]

Awesome, thank you so much!

[lilyinstarlight]

I'll merge this when rebuilds finish, thanks again :)
(It could be a while given it looks like the channel bump rebuilds all cosmic apps...)