Update dependency optimize-css-assets-webpack-plugin to v6

Author: renovate[bot]

package.json

@@ -48,7 +48,7 @@
     "netlify-cms-app": "^2.15.72",
     "node-sass": "^4.10.0",
     "npm-run-all": "^4.1.5",
-    "optimize-css-assets-webpack-plugin": "^5.0.1",
+    "optimize-css-assets-webpack-plugin": "^6.0.0",
     "plugin-error": "^1.0.1",
     "postcss-import": "^12.0.1",
     "postcss-loader": "^3.0.0",