Bump @fullhuman/postcss-purgecss from 5.0.0 to 6.0.0

Bumps [@fullhuman/postcss-purgecss](https://github.com/FullHuman/purgecss) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/FullHuman/purgecss/releases)
- [Changelog](https://github.com/FullHuman/purgecss/blob/main/CHANGELOG.md)
- [Commits](FullHuman/purgecss@v5.0.0...v6.0.0)

---
updated-dependencies:
- dependency-name: "@fullhuman/postcss-purgecss"
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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