Bump next from 14.2.15 to 14.2.21 in /examples/nextjs

Bumps [next](https://github.com/vercel/next.js) from 14.2.15 to 14.2.21.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v14.2.15...v14.2.21)

---
updated-dependencies:
- dependency-name: next
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

examples/nextjs/package.json

@@ -13,7 +13,7 @@
     "@xennis/react-notion-cms-fetch": "workspace:*",
     "@xennis/react-notion-cms-render": "workspace:*",
     "highlight.js": "^11.11.1",
-    "next": "14.2.15",
+    "next": "14.2.21",
     "prismjs": "^1.29.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1"