nixos/xterm: Set default to xserver.enable

[alexarice]

Motivation for this change

Fixes #69079
I bisected this issue down to 456c42c
This changes the default for xterm to be true if stateVersion is older than 19.09.
I believe that in this case the default should be set to the xserver.enable default, as it was 2 commits before.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @worldofpeace

[matthewbauer]

We may want to just enable xterm on those tests that need it.

[alexarice]

The problem is that it is enabled when it shouldn't be (i.e. when xserver is not enabled). The tests don't need it

[matthewbauer]

The problem is that it is enabled when it shouldn't be (i.e. when xserver is not enabled). The tests don't need it

Ah ok! My bad, I misunderstood what the problem.

[worldofpeace]

Yeah this is what I meant to fix after it got merged a bit back, but I forgot about it 🤣

I actually want a fix like #68371, but I'm not too sure about the PR because I don't understand what we need to support.

[FRidh]

Let's get this in first to unblock the channels, if the fix is acceptable.

[worldofpeace]

LGTM