Upgrade cssnano to v6.0.0 (#4233)

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
...

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

{{cookiecutter.project_slug}}/package.json

@@ -12,7 +12,7 @@
     "css-loader": "^6.5.1",
     "gulp-concat": "^2.6.1",
     "concurrently": "^8.0.1",
-    "cssnano": "^5.0.11",
+    "cssnano": "^6.0.0",
     "gulp": "^4.0.2",
     "gulp-imagemin": "^7.1.0",
     "gulp-plumber": "^1.2.1",