Bump cssnano from 5.1.15 to 6.0.1

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.1.15 to 6.0.1.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@5.1.15...cssnano@6.0.1)

---
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

@@ -49,7 +49,7 @@
         "babelify": "^10.0.0",
         "browserify": "^17.0.0",
         "cross-env": "^7.0.3",
-        "cssnano": "^5.1.15",
+        "cssnano": "^6.0.1",
         "del": "^6.1.1",
         "factor-bundle": "^2.5.0",
         "gulp": "^4.0.2",