Bump optimize-css-assets-webpack-plugin from 5.0.4 to 6.0.0

Bumps [optimize-css-assets-webpack-plugin](https://github.com/NMFR/optimize-css-assets-webpack-plugin) from 5.0.4 to 6.0.0.
- [Release notes](https://github.com/NMFR/optimize-css-assets-webpack-plugin/releases)
- [Commits](NMFR/optimize-css-assets-webpack-plugin@v5.0.4...v6.0.0)

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package.json

@@ -235,7 +235,7 @@
     "lint-staged": "^10.2.11",
     "mini-css-extract-plugin": "1.3.1",
     "opencollective-postinstall": "^2.0.3",
-    "optimize-css-assets-webpack-plugin": "^5.0.3",
+    "optimize-css-assets-webpack-plugin": "^6.0.0",
     "prettier": "^2.2.1",
     "react-test-renderer": "17.0.1",
     "redux-logger": "^3.0.6",