chore(deps): update dependency cssnano to v7

Author: renovate[bot]

docsSrc/package.json

@@ -36,7 +36,7 @@
     "autoprefixer": "^10.4.7",
     "babel-eslint": "^10.1.0",
     "broccoli-asset-rev": "^3.0.0",
-    "cssnano": "^5.1.11",
+    "cssnano": "^7.0.0",
     "ember-auto-import": "^2.4.2",
     "ember-build-prerender": "^0.3.2",
     "ember-cli": "~4.8.0",