Bump postcss and autoprefixer

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.18 and updates ancestor dependency [autoprefixer](https://github.com/postcss/autoprefixer). These dependencies need to be updated together.


Updates `postcss` from 6.0.23 to 8.4.18
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...8.4.18)

Updates `autoprefixer` from 8.6.5 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@8.6.5...10.4.13)

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

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