Bump clean-css from 3.2.2 to 4.2.1 in /tests/functional/cssApp

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 3.2.2 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@v3.2.2...v4.2.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

tests/functional/cssApp/package.json

@@ -3,7 +3,7 @@
   "version": "0.1.0",
   "description": "Test application using css! plugin",
   "devDependencies": {
-    "clean-css": "3.2.2",
+    "clean-css": "4.2.1",
     "grunt-amd-build": "0.10.0-alpha",
     "grunt-contrib-clean": "0.6.x",
     "grunt-contrib-concat": "0.5.x",