Bump clean-css from 4.1.8 to 4.2.3

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 4.1.8 to 4.2.3.
- [Release notes](https://github.com/jakubpawlowicz/clean-css/releases)
- [Changelog](https://github.com/jakubpawlowicz/clean-css/blob/v4.2.3/History.md)
- [Commits](https://github.com/jakubpawlowicz/clean-css/compare/v4.1.8...v4.2.3

)

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