Bump postcss and stylelint

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [stylelint](https://github.com/stylelint/stylelint). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.31)

Updates `stylelint` from 13.13.1 to 15.10.3
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/main/CHANGELOG.md)
- [Commits](stylelint/stylelint@13.13.1...15.10.3)

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

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