BREAKING CHANGE: Update dependency postcss-loader to v4

Author: renovate-bot

package.json

@@ -49,7 +49,7 @@
     "object-assign": "4.1.1",
     "optimize-css-assets-webpack-plugin": "5.0.3",
     "postcss-flexbugs-fixes": "4.2.1",
-    "postcss-loader": "3.0.0",
+    "postcss-loader": "4.0.3",
     "postcss-safe-parser": "4.0.2",
     "promise": "8.1.0",
     "prompt": "1.0.0",