build(deps): bump next from 15.5.18 to 15.5.21 (#7372)

Bumps [next](https://github.com/vercel/next.js) from 15.5.18 to 15.5.21.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Commits](https://github.com/vercel/next.js/compare/v15.5.18...v15.5.21)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.21
  dependency-type: direct:production
...

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document/package.json

@@ -28,7 +28,7 @@
     "gray-matter": "^4.0.3",
     "lucide-react": "^0.525.0",
     "mermaid": "^10.9.6",
-    "next": "^15.5.18",
+    "next": "^15.5.21",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "react-responsive": "^10.0.1",