Bump postcss, stylelint and stylelint-order

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.21 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [stylelint](https://github.com/stylelint/stylelint) and [stylelint-order](https://github.com/hudochenkov/stylelint-order). These dependencies need to be updated together.


Updates `postcss` from 5.2.18 to 8.4.21
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/commits/8.4.21)

Updates `stylelint` from 8.2.0 to 15.2.0
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/main/CHANGELOG.md)
- [Commits](stylelint/stylelint@8.2.0...15.2.0)

Updates `stylelint-order` from 0.7.0 to 6.0.2
- [Release notes](https://github.com/hudochenkov/stylelint-order/releases)
- [Changelog](https://github.com/hudochenkov/stylelint-order/blob/master/CHANGELOG.md)
- [Commits](hudochenkov/stylelint-order@0.7.0...6.0.2)

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

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

Author: dependabot[bot]