chore(deps): bump next from 14.2.14 to 14.2.21 in /demo/nextjs

Bumps [next](https://github.com/vercel/next.js) from 14.2.14 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@v14.2.14...v14.2.21)

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

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

demo/nextjs/package.json

@@ -9,7 +9,7 @@
 		"lint": "next lint"
 	},
 	"dependencies": {
-		"next": "14.2.14",
+		"next": "14.2.21",
 		"react": "^18.3.1",
 		"react-dom": "^18.3.1"
 	},