git: change stateVersion check for compatiblity

[khaneliman]

Original change was created before 24.11 release. We are in 25.05 now.

Follow up to #5516

Description

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

[sumnerevans]

Whoops, thanks!

[wzxu]

I'm still not quite sure how stateVersion works, but I'm currently using stateVersion = "25.05" and I suppose this would fix my failing build due to error: The option `home-manager.users.wzxu.programs.git.signing.format' was accessed but has no value defined. Try setting the option., but it still fails the same.

Do I have to set this option even if I'm not using signing? Or should I not use stateVersion = "25.05" at all? Thanks.