Bump stylelint-scss from 3.18.0 to 3.21.0

Bumps [stylelint-scss](https://github.com/kristerkari/stylelint-scss) from 3.18.0 to 3.21.0.
- [Release notes](https://github.com/kristerkari/stylelint-scss/releases)
- [Changelog](https://github.com/kristerkari/stylelint-scss/blob/master/CHANGELOG.md)
- [Commits](stylelint-scss/stylelint-scss@3.18.0...3.21.0)

---
updated-dependencies:
- dependency-name: stylelint-scss
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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