build: Bump postcss and css-loader

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.23 and updates ancestor dependency [css-loader](https://github.com/webpack-contrib/css-loader). These dependencies need to be updated together.


Updates `postcss` from 5.2.18 to 8.4.23
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/commits/8.4.23)

Updates `css-loader` from 0.23.1 to 6.7.3
- [Release notes](https://github.com/webpack-contrib/css-loader/releases)
- [Changelog](https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/css-loader@v0.23.1...v6.7.3)

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

Signed-off-by: dependabot[bot] <support@github.com>