Bump next from 14.2.15 to 14.2.21 in /references/nextjs-realtime (#1583)

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](https://github.com/vercel/next.js/compare/v14.2.15...v14.2.21)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

references/nextjs-realtime/package.json

@@ -30,7 +30,7 @@
     "date-fns": "^4.1.0",
     "langsmith": "^0.2.15",
     "lucide-react": "^0.451.0",
-    "next": "14.2.15",
+    "next": "14.2.21",
     "openai": "^4.68.4",
     "react": "^18",
     "react-dom": "^18",