chore(deps): Bump postcss and cssnano

Bumps [postcss](https://github.com/postcss/postcss) and [cssnano](https://github.com/cssnano/cssnano). These dependencies needed to be updated together.

Updates `postcss` from 8.4.26 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@8.4.26...8.4.31)

Updates `cssnano` from 4.1.11 to 6.0.1
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/v4.1.11...cssnano@6.0.1)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: cssnano
  dependency-type: indirect
...

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