Bump next from 15.1.6 to 15.2.3 in /demos/toolkit/next

Bumps [next](https://github.com/vercel/next.js) from 15.1.6 to 15.2.3.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.1.6...v15.2.3)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

demos/toolkit/next/package.json

@@ -14,7 +14,7 @@
     "@types/react": "19.0.8",
     "@types/react-dom": "19.0.3",
     "@yfiles/demo-resources": "../../resources",
-    "next": "15.1.6",
+    "next": "15.2.3",
     "react": "19.0.0",
     "react-dom": "19.0.0",
     "@yfiles/yfiles": "../../../lib/yfiles-30.0.0+dev.tgz"