Replace Less with CSS (#23508)

Backport https://github.com/go-gitea/gitea/pull/23481,
https://github.com/go-gitea/gitea/pull/23504 and
https://github.com/go-gitea/gitea/pull/23520 to 1.19, just so we have an
easier time with future backports.

Seems to work on a basic level. There was a merge conflict in
`RepoActionView.vue`, otherwise it merged cleanly.

---------

Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

.stylelintrc.yaml

@@ -5,8 +5,6 @@ ignoreFiles:
   - "**/*.go"
 
 overrides:
-  - files: ["**/*.less"]
-    customSyntax: postcss-less
   - files: ["**/chroma/*", "**/codemirror/*", "**/standalone/*", "**/console/*"]
     rules:
       scale-unlimited/declaration-strict-value: null