Bump clean-css-cli from 4.3.0 to 5.1.0

Bumps [clean-css-cli](https://github.com/jakubpawlowicz/clean-css-cli) from 4.3.0 to 5.1.0.
- [Release notes](https://github.com/jakubpawlowicz/clean-css-cli/releases)
- [Changelog](https://github.com/jakubpawlowicz/clean-css-cli/blob/master/History.md)
- [Commits](https://github.com/jakubpawlowicz/clean-css-cli/compare/v4.3.0...v5.1.0)

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

package.json

@@ -59,7 +59,7 @@
     "buble": "^0.20.0",
     "buble-loader": "^0.5.1",
     "chaikin-smooth": "^1.0.4",
-    "clean-css-cli": "4.3.0",
+    "clean-css-cli": "5.1.0",
     "copy-webpack-plugin": "^6.0.1",
     "coveralls": "3.1.0",
     "eslint": "^7.2.0",