fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-NEXT-9508709

Author: snyk-bot

package.json

@@ -13,7 +13,7 @@
   "dependencies": {
     "@mdx-js/loader": "^1.6.22",
     "@next/mdx": "^11.1.0",
-    "next": "12",
+    "next": "12.3.5",
     "react": "^17.0.2",
     "react-dom": "^17.0.2",
     "sass": "^1.38.0"