Bump postcss and react-scripts

Bumps [postcss](https://github.com/postcss/postcss) and [react-scripts](https://github.com/facebook/create-react-app/tree/HEAD/packages/react-scripts). These dependencies needed to be updated together.

Updates `postcss` from 7.0.21 to 8.4.16
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.21...8.4.16)

Updates `react-scripts` from 3.4.4 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-3.x.md)
- [Commits](https://github.com/facebook/create-react-app/commits/[email protected]/packages/react-scripts)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: react-scripts
  dependency-type: direct:production
...

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

Author: dependabot[bot]