chore(deps-dev): bump cssnano from 5.1.15 to 6.0.0 (#8)

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

@@ -51,7 +51,7 @@
     "@lodder/grunt-postcss": "^3.1.1",
     "autoprefixer": "^10.4.13",
     "bumpp": "^9.0.0",
-    "cssnano": "^5.1.15",
+    "cssnano": "^6.0.0",
     "grunt": "^1.6.1",
     "grunt-sass": "^3.1.0",
     "load-grunt-tasks": "^5.1.0",