Bump clean-css from 4.1.9 to 4.1.11

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 4.1.9 to 4.1.11.
- [Release notes](https://github.com/jakubpawlowicz/clean-css/releases)
- [Changelog](https://github.com/jakubpawlowicz/clean-css/blob/master/History.md)
- [Commits](clean-css/clean-css@v4.1.9...v4.1.11)

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