Bump @primer/css from 17.9.0 to 21.3.6

Bumps [@primer/css](https://github.com/primer/css) from 17.9.0 to 21.3.6.
- [Release notes](https://github.com/primer/css/releases)
- [Changelog](https://github.com/primer/css/blob/main/CHANGELOG.md)
- [Commits](primer/css@v17.9.0...v21.3.6)

---
updated-dependencies:
- dependency-name: "@primer/css"
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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