Update dependency postcss to v8.5.6

Author: renovate[bot]

package-lock.json

@@ -21,7 +21,7 @@
     "css-loader": "7.1.2",
     "html-webpack-plugin": "5.6.3",
     "mini-css-extract-plugin": "2.9.2",
-    "postcss": "8.5.5",
+    "postcss": "8.5.6",
     "postcss-loader": "8.1.1",
     "sass": "1.89.2",
     "sass-loader": "16.0.5",