Bump css-loader from 4.3.0 to 5.1.0 (#6735)

Bumps [css-loader](https://github.com/webpack-contrib/css-loader) from 4.3.0 to 5.1.0.
- [Release notes](https://github.com/webpack-contrib/css-loader/releases)
- [Changelog](https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/css-loader@v4.3.0...v5.1.0)

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package.json

@@ -128,7 +128,7 @@
     "commander": "6.2.1",
     "concurrently": "^6.0.0",
     "cspell": "^5.1.3",
-    "css-loader": "^4.3.0",
+    "css-loader": "^5.1.0",
     "dotenv": "^8.2.0",
     "ejs": "^3.1.6",
     "github-api": "^3.4.0",