fix: upgrade postcss from 8.4.38 to 8.5.6

Snyk has created this PR to upgrade postcss from 8.4.38 to 8.5.6.

See this package in npm:
postcss

See this project in Snyk:
https://app.snyk.io/org/ivan-QBnRiSBZoUxnTVY6mQXyuA/project/a3c60ec2-739f-4e92-ad5f-338635aebcd0?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot

package.json

@@ -11,7 +11,7 @@
     "next-sitemap": "4.2.3",
     "nextra": "2.13.4",
     "nextra-theme-docs": "2.13.4",
-    "postcss": "8.4.38",
+    "postcss": "8.5.6",
     "react": "18.3.1",
     "react-dom": "18.3.1",
     "tailwindcss": "3.4.3",