Bump postcss and typescript-plugin-css-modules in /frontend

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.21 and updates ancestor dependency [typescript-plugin-css-modules](https://github.com/mrmckeb/typescript-plugin-css-modules). These dependencies need to be updated together.


Updates `postcss` from 6.0.23 to 8.4.21
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...8.4.21)

Updates `typescript-plugin-css-modules` from 3.4.0 to 4.2.2
- [Release notes](https://github.com/mrmckeb/typescript-plugin-css-modules/releases)
- [Commits](mrmckeb/typescript-plugin-css-modules@v3.4.0...v4.2.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: typescript-plugin-css-modules
  dependency-type: direct:production
...

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