Bump postcss, css-loader and postcss-loader (#46)

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.19 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [css-loader](https://github.com/webpack-contrib/css-loader) and [postcss-loader](https://github.com/webpack-contrib/postcss-loader). These dependencies need to be updated together.


Updates `postcss` from 7.0.35 to 8.4.19
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.35...8.4.19)

Updates `css-loader` from 0.28.11 to 6.7.2
- [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@v0.28.11...v6.7.2)

Updates `postcss-loader` from 2.1.6 to 7.0.2
- [Release notes](https://github.com/webpack-contrib/postcss-loader/releases)
- [Changelog](https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](webpack/postcss-loader@v2.1.6...v7.0.2)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]