Update dependency postcss to v8.4.21

Author: renovate[bot]

package-lock.json

@@ -26,7 +26,7 @@
  "jsdom": "20.0.3",
  "laravel-mix": "6.0.49",
  "lodash": "4.17.21",
- "postcss": "8.4.20",
+ "postcss": "8.4.21",
  "tailwindcss": "3.2.4",
  "vue-loader": "17.0.1"
  },