Update mini-css-extract-plugin, remove postcss (#23520)

Follow-up and proper fix for
https://github.com/go-gitea/gitea/pull/23504

Update to
[mini-css-extract-plugin@2.7.4](https://github.com/webpack-contrib/mini-css-extract-plugin/releases/tag/v2.7.4)
which fixes our specific issue described in
https://github.com/webpack-contrib/css-loader/issues/1503 and which
allows us to again drop the postcss dependency.

Backport of this is not necessary as I have included it in
https://github.com/go-gitea/gitea/pull/23508.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

webpack.config.js

@@ -145,23 +145,10 @@ export default {
             loader: 'css-loader',
             options: {
               sourceMap: true,
-              importLoaders: 1,
               url: {filter: filterCssImport},
               import: {filter: filterCssImport},
             },
           },
-          {
-            loader: 'postcss-loader', /* for conditional import in theme-auto.css */
-            options: {
-              sourceMap: true,
-              postcssOptions: {
-                plugins: [
-                  'postcss-import',
-                  'postcss-url',
-                ],
-              },
-            },
-          },
         ],
       },
       {