Use esbuild to minify CSS (#15756)

It's about a 30% speedup in webpack build time with neglible differences
in the output size. We do lose the ability for CSS source maps, but I
rarely have a use for them anyways.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: 6543 <6543@obermui.de>

package.json

@@ -12,7 +12,6 @@
     "clipboard": "2.0.8",
     "codemirror": "5.61.0",
     "css-loader": "5.2.4",
-    "css-minimizer-webpack-plugin": "2.0.0",
     "dropzone": "5.9.2",
     "easymde": "2.14.0",
     "esbuild-loader": "2.12.0",
@@ -28,7 +27,6 @@
     "mini-css-extract-plugin": "1.5.0",
     "monaco-editor": "0.23.0",
     "monaco-editor-webpack-plugin": "3.0.1",
-    "postcss": "8.2.12",
     "pretty-ms": "7.0.1",
     "sortablejs": "1.13.0",
     "swagger-ui-dist": "3.47.1",