build(deps): bump next from 15.5.18 to 15.5.21

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](vercel/next.js@v15.5.18...v15.5.21)

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

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

Author: dependabot[bot]

apps/studio/package.json

@@ -87,7 +87,7 @@
     "mermaid": "^11.15.0",
     "monaco-editor": "0.34.1",
     "monaco-yaml": "4.0.2",
-    "next": "15.5.18",
+    "next": "15.5.21",
     "postcss": "8.4.31",
     "react": "18.2.0",
     "react-dom": "18.2.0",