Bump next from 15.2.4 to 15.5.6

Bumps [next](https://github.com/vercel/next.js) from 15.2.4 to 15.5.6.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.2.4...v15.5.6)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.6
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

frontend/package.json

@@ -10,7 +10,7 @@
     "jwt-decode": "^4.0.0",
     "markdown-it": "^13.0.2",
     "markdown-to-jsx": "^7.7.10",
-    "next": "^15.2.3",
+    "next": "^15.5.6",
     "next-client-cookies": "^2.0.1",
     "nhs-notify-backend-client": "^0.0.1",
     "nhs-notify-web-template-management-utils": "^0.0.1",