Bump clean-css from 3.4.28 to 4.2.1

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 3.4.28 to 4.2.1.
- [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@v3.4.28...v4.2.1)

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

Author: dependabot[bot]

package.json

@@ -20,7 +20,7 @@
     "node": ">=0.10"
   },
   "dependencies": {
-    "clean-css": "^3.0.1"
+    "clean-css": "^4.2.1"
   },
   "keywords": [
     "less plugins",