build(deps): bump postcss, autoprefixer, css-loader, postcss-flexbugs-fixes and postcss-loader

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.21 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [autoprefixer](https://github.com/postcss/autoprefixer), [css-loader](https://github.com/webpack-contrib/css-loader), [postcss-flexbugs-fixes](https://github.com/luisrudge/postcss-flexbugs-fixes) and [postcss-loader](https://github.com/webpack-contrib/postcss-loader). These dependencies need to be updated together.


Updates `postcss` from 5.2.18 to 8.4.21
- [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/8.4.21)

Updates `autoprefixer` from 7.1.6 to 10.4.13
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@7.1.6...10.4.13)

Updates `css-loader` from 0.28.7 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/css-loader@v0.28.7...v6.7.3)

Updates `postcss-flexbugs-fixes` from 3.2.0 to 5.0.2
- [Release notes](https://github.com/luisrudge/postcss-flexbugs-fixes/releases)
- [Changelog](https://github.com/luisrudge/postcss-flexbugs-fixes/blob/master/CHANGELOG.md)
- [Commits](https://github.com/luisrudge/postcss-flexbugs-fixes/commits/5.0.2)

Updates `postcss-loader` from 2.0.8 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@v2.0.8...v7.0.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: autoprefixer
  dependency-type: direct:production
- dependency-name: css-loader
  dependency-type: direct:production
- dependency-name: postcss-flexbugs-fixes
  dependency-type: direct:production
- dependency-name: postcss-loader
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]