Bump postcss and stylelint-webpack-plugin in /client

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [stylelint-webpack-plugin](https://github.com/webpack-contrib/stylelint-webpack-plugin). 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-webpack-plugin` from 3.1.0 to 3.3.0
- [Release notes](https://github.com/webpack-contrib/stylelint-webpack-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/stylelint-webpack-plugin/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/stylelint-webpack-plugin@v3.1.0...v3.3.0)

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

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