Bump next from 15.3.5 to 15.4.7 in /packages/gitbook

Bumps [next](https://github.com/vercel/next.js) from 15.3.5 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@v15.3.5...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]

packages/gitbook/package.json

@@ -46,7 +46,7 @@
         "memoizee": "^0.4.17",
         "micromark-extension-frontmatter": "^2.0.0",
         "micromark-extension-gfm": "^3.0.0",
-        "next": "15.3.5",
+        "next": "15.4.7",
         "next-themes": "^0.2.1",
         "nuqs": "^2.2.3",
         "object-hash": "^3.0.0",