Bump next from 15.4.0 to 15.4.8 in /packages/gitbook

Bumps [next](https://github.com/vercel/next.js) from 15.4.0 to 15.4.8.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.4.0...v15.4.8)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.4.8
  dependency-type: direct:production
...

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

Author: dependabot[bot]

packages/gitbook/package.json

@@ -50,7 +50,7 @@
         "micromark-extension-frontmatter": "^2.0.0",
         "micromark-extension-gfm": "^3.0.0",
         "motion": "^12.23.24",
-        "next": "15.4.0",
+        "next": "15.4.8",
         "next-themes": "^0.4.6",
         "nuqs": "^2.2.3",
         "object-hash": "^3.0.0",