chore(deps): bump postcss, react-scripts and rollup-plugin-postcss

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.21 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [react-scripts](https://github.com/facebook/create-react-app/tree/HEAD/packages/react-scripts) and [rollup-plugin-postcss](https://github.com/egoist/rollup-plugin-postcss). 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 `react-scripts` from 2.1.8 to 5.0.1
- [Release notes](https://github.com/facebook/create-react-app/releases)
- [Changelog](https://github.com/facebook/create-react-app/blob/main/CHANGELOG-2.x.md)
- [Commits](https://github.com/facebook/create-react-app/commits/[email protected]/packages/react-scripts)

Updates `rollup-plugin-postcss` from 2.9.0 to 4.0.2
- [Release notes](https://github.com/egoist/rollup-plugin-postcss/releases)
- [Changelog](https://github.com/egoist/rollup-plugin-postcss/blob/master/CHANGELOG-OLD.md)
- [Commits](https://github.com/egoist/rollup-plugin-postcss/commits/v4.0.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: react-scripts
  dependency-type: direct:development
- dependency-name: rollup-plugin-postcss
  dependency-type: direct:development
...

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