security: bump next to 15.2.3 in next-js-15 example

Addresses critical advisories for Next.js 15.x (middleware bypass
GHSA-f82v-jwr5-mffw and React flight RCE GHSA-9qr9-h5gf-34mp).

Made-with: Cursor

Author: thomasrockhu-codecov

examples/next-js-15/package.json

@@ -9,7 +9,7 @@
     "lint": "next lint"
   },
   "dependencies": {
-    "next": "15.1.0",
+    "next": "15.2.3",
     "react": "19.0.0",
     "react-dom": "19.0.0"
   },