Upgrade clean-css to 5.x

The css minifier should not strip the units on zeros when they are being used in a css math function. This has been fixed in clean-css 5.x (see https://github.com/clean-css/clean-css/issues/1192).

Just requesting a new version that includes this upgrade :)

Locate the repository's clean-css dependency declaration and check how dependency versions are managed. Upgrade it to a 5.x release that includes the referenced fix, then run the project's existing tests or CSS-minification checks to confirm units on zeros inside CSS math functions are preserved.

Written by the indexing model from the issue text.