Bump cssnano from 5.1.15 to 6.0.0 (#2313)

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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -62,7 +62,7 @@
     "babel-plugin-transform-es2015-modules-commonjs": "^6.26.2",
     "chai": "^4.3.7",
     "codeclimate-test-reporter": "^0.5.1",
-    "cssnano": "^5.1.15",
+    "cssnano": "^6.0.0",
     "cypress": "12.9.0",
     "cypress-plugin-tab": "^1.0.5",
     "del": "^7.0.0",