Update dependency postcss to v8.5.6

Author: renovate[bot]

package.json

@@ -13,6 +13,6 @@
         "axios": "^0.26.0",
         "laravel-mix": "6.0.43",
         "lodash": "4.17.21",
-        "postcss": "8.4.8"
+        "postcss": "8.5.6"
     }
 }