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

Follow-up and proper fix for
#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
webpack-contrib/css-loader#1503 and which
allows us to again drop the postcss dependency.

Backport of this is not necessary as I have included it in
#23508.

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