Bump stylelint from 13.8.0 to 13.12.0 in /frontend

Bumps [stylelint](https://github.com/stylelint/stylelint) from 13.8.0 to 13.12.0.
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/master/CHANGELOG.md)
- [Commits](stylelint/stylelint@13.8.0...13.12.0)

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