Bump cssnano from 5.1.15 to 6.0.0

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.1.15 to 6.0.0.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@5.1.15...cssnano@6.0.0)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

package.json

@@ -15,7 +15,7 @@
         "browserlist": "^1.0.1",
         "compression-webpack-plugin": "^10.0.0",
         "css-loader": "^6.7.3",
-        "cssnano": "^5.1.15",
+        "cssnano": "^6.0.0",
         "eslint": "^8.37.0",
         "eslint-webpack-plugin": "^4.0.0",
         "mini-css-extract-plugin": "^2.7.5",