treewide: deprecate manual targets.${target}.useWallpaper.enable options

[trueNAHO]

Deprecate manual targets.${target}.useWallpaper.enable options with
generated targets.${target}.image.enable options, following commit
953c3fb01e2a ("stylix/mk-target: generate options for configuring
safeguarded arguments").

The following remaining useWallpaper instances cannot yet be deprecated because their modules have not yet migrated to mkTarget:

$ rg --files-with-matches 'useWallpaper ='
modules/grub/nixos.nix
modules/kde/hm.nix
modules/sway/hm.nix

I have not tested this code and merely assume this works as expected. This has been tested in #2084 (review).

---

• I certify that I have the right to submit this contribution under the MIT license
• Commit messages adhere to Stylix commit conventions
• Theming changes adhere to the Stylix style guide
• Changes have been tested locally
• Changes have been tested in testbeds
• Each commit in this PR is suitable for backport to the current stable branch

[0xda157]

otherwise LGTM

[trueNAHO]

I have not tested this code and merely assume this works as expected.

The following tests successfully build:

• diff --git a/modules/regreet/testbeds/regreet.nix b/modules/regreet/testbeds/regreet.nix
  index a4ec92ae..618b32fc 100644
  --- a/modules/regreet/testbeds/regreet.nix
  +++ b/modules/regreet/testbeds/regreet.nix
  @@ -1 +1,4 @@
  -{ programs.regreet.enable = true; }
  +{
  +  stylix.targets.regreet.useWallpaper = true;
  +  programs.regreet.enable = true;
  +}

• diff --git a/modules/regreet/testbeds/regreet.nix b/modules/regreet/testbeds/regreet.nix
  index a4ec92ae..388498ee 100644
  --- a/modules/regreet/testbeds/regreet.nix
  +++ b/modules/regreet/testbeds/regreet.nix
  @@ -1 +1,4 @@
  -{ programs.regreet.enable = true; }
  +{
  +  stylix.targets.regreet.useWallpaper = false;
  +  programs.regreet.enable = true;
  +}