fix(deps): update dependency postcss to v8.4.21

Author: renovate[bot]

package-lock.json

@@ -48,7 +48,7 @@
     "next-seo": "4.29.0",
     "next-sitemap": "1.9.12",
     "next-transpile-modules": "8.0.0",
-    "postcss": "8.4.20",
+    "postcss": "8.4.21",
     "prop-types": "15.8.1",
     "react": "17.0.2",
     "react-copy-to-clipboard": "5.1.0",