Bump rouge from 3.20.0 to 3.21.0

Bumps [rouge](https://github.com/rouge-ruby/rouge) from 3.20.0 to 3.21.0.
- [Release notes](https://github.com/rouge-ruby/rouge/releases)
- [Changelog](https://github.com/rouge-ruby/rouge/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rouge-ruby/rouge/compare/v3.20.0...v3.21.0)