chore: downgrade Next.js to 15.3.2 to support open-next adapter

Author: dario-piotrowicz

apps/site/package.json

@@ -57,7 +57,7 @@
     "feed": "~5.1.0",
     "github-slugger": "~2.0.0",
     "gray-matter": "~4.0.3",
-    "next": "15.4.3",
+    "next": "15.3.2",
     "next-intl": "~4.3.4",
     "next-themes": "~0.4.6",
     "postcss-calc": "~10.1.1",