Bump postcss and newman

Removes [postcss](https://github.com/postcss/postcss). It's no longer used after updating ancestor dependency [newman](https://github.com/postmanlabs/newman). These dependencies need to be updated together.


Removes `postcss`

Updates `newman` from 4.5.7 to 6.0.0
- [Changelog](https://github.com/postmanlabs/newman/blob/develop/CHANGELOG.yaml)
- [Commits](postmanlabs/newman@v4.5.7...v6.0.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: newman
  dependency-type: direct:development
...

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