chore: downgrade next on middleware demo

Author: pieh

demos/middleware/package.json

@@ -12,7 +12,7 @@
     "@netlify/next": "*",
     "@netlify/plugin-nextjs": "*",
     "isomorphic-unfetch": "^3.1.0",
-    "next": "^13.4.1",
+    "next": "13.2.4",
     "react": "^18.2.0",
     "react-dom": "^18.2.0"
   },