mini-css-extract-plugin is totally outdated

[stof]

We currently force installing at most mini-css-extract-plugin@0.4.2, which is from 2018. Latest version is 0.9.0.

[Lyrkan]

I already tried to update it in #564 but for some reason it came with inconsistent hashes in filenames.

It seems to be fixed when used with Webpack 5 so it will probably be updated once #645 is released.

[stof]

this has been updated in the meantime