Bump color-string and css-loader

Removes [color-string](https://github.com/Qix-/color-string). It's no longer used after updating ancestor dependency [css-loader](https://github.com/webpack-contrib/css-loader). These dependencies need to be updated together.


Removes `color-string`

Updates `css-loader` from 0.28.7 to 6.7.1
- [Release notes](https://github.com/webpack-contrib/css-loader/releases)
- [Changelog](https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md)
- [Commits](webpack/css-loader@v0.28.7...v6.7.1)

---
updated-dependencies:
- dependency-name: color-string
  dependency-type: indirect
- dependency-name: css-loader
  dependency-type: direct:development
...

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Author: dependabot[bot]