chore(deps): bump next in /packages/react-sdk/dev/nextjs-flag-demo

Bumps [next](https://github.com/vercel/next.js) from 15.5.10 to 15.5.15.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.5.10...v15.5.15)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.15
  dependency-type: direct:production
...

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

Author: dependabot[bot]

packages/react-sdk/dev/nextjs-flag-demo/package.json

@@ -15,7 +15,7 @@
   },
   "dependencies": {
     "@reflag/react-sdk": "workspace:^",
-    "next": "15.5.10",
+    "next": "15.5.15",
     "react": "^18",
     "react-dom": "^18"
   },