Bump postcss and react-scripts in /flowauth/frontend

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.14 and updates ancestor dependency [react-scripts](https://github.com/facebook/create-react-app/tree/HEAD/packages/react-scripts). These dependencies need to be updated together.


Updates `postcss` from 7.0.36 to 8.5.14
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.36...8.5.14)

Updates `react-scripts` from 4.0.3 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-4.x.md)
- [Commits](https://github.com/facebook/create-react-app/commits/react-scripts@5.0.1/packages/react-scripts)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.13
  dependency-type: indirect
- dependency-name: react-scripts
  dependency-version: 5.0.1
  dependency-type: direct:production
...

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Author: dependabot[bot]