Bump next from 15.1.7 to 15.4.7 in /samples/mastra-nextjs/app

Bumps [next](https://github.com/vercel/next.js) from 15.1.7 to 15.4.7.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.1.7...v15.4.7)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.4.7
  dependency-type: direct:production
...

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

Author: dependabot[bot]

samples/mastra-nextjs/app/package.json

@@ -29,7 +29,7 @@
     "dotenv": "^16.4.7",
     "drizzle-orm": "^0.39.3",
     "lucide-react": "^0.475.0",
-    "next": "15.1.7",
+    "next": "15.4.7",
     "next-safe-action": "^7.10.3",
     "next-themes": "^0.4.4",
     "octokit": "^4.1.1",