Update dependency postcss to v8.4.20

Author: renovate[bot]

package-lock.json

@@ -20,7 +20,7 @@
     "css-loader": "6.7.2",
     "html-webpack-plugin": "5.5.0",
     "mini-css-extract-plugin": "2.7.2",
-    "postcss": "8.4.19",
+    "postcss": "8.4.20",
     "postcss-loader": "7.0.2",
     "sass": "1.56.2",
     "sass-loader": "13.2.0",