Upgrade webpack's css-loader to 6.x

[indeyets]

We use modern webpack, but CSS-loader is still on 5.x. We should update it (current version is 6.7.1)

CSS-loader 6.x uses modern facilities of webpack and should work faster.

The problem is, that it has a lot of breaking changes https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md#600-2021-07-14 and it is possible that it would require some additional changes to our codebase (such as removal of file-loader, url-loader, changing resource-templates etc.)