Bump postcss, autoprefixer, cssnano and postcss-flexbugs-fixes

Bumps [postcss](https://github.com/postcss/postcss), [autoprefixer](https://github.com/postcss/autoprefixer), [cssnano](https://github.com/cssnano/cssnano) and [postcss-flexbugs-fixes](https://github.com/luisrudge/postcss-flexbugs-fixes). These dependencies needed to be updated together.

Updates `postcss` from 6.0.23 to 8.4.47
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...8.4.47)

Updates `autoprefixer` from 7.2.6 to 10.4.20
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@7.2.6...10.4.20)

Updates `cssnano` from 3.10.0 to 7.0.6
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/v3.10.0...cssnano@7.0.6)

Updates `postcss-flexbugs-fixes` from 3.3.1 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)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:development
- dependency-name: autoprefixer
  dependency-type: direct:development
- dependency-name: cssnano
  dependency-type: direct:development
- dependency-name: postcss-flexbugs-fixes
  dependency-type: direct:development
...

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

Author: dependabot[bot]