Bump next from 15.3.3 to 15.5.4

Bumps [next](https://github.com/vercel/next.js) from 15.3.3 to 15.5.4.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.3.3...v15.5.4)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

frontend/package.json

@@ -28,7 +28,7 @@
     "js-cookie": "^3.0.5",
     "jwt-decode": "^4.0.0",
     "markdown-to-jsx": "^7.7.13",
-    "next": "^15.3.2",
+    "next": "^15.5.4",
     "nhsuk-frontend": "^9.3.0",
     "nhsuk-react-components": "^5.0.0",
     "path": "^0.12.7",