Bump next from 14.2.13 to 14.2.15 in /frontend

Bumps [next](https://github.com/vercel/next.js) from 14.2.13 to 14.2.15.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v14.2.13...v14.2.15)

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

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

Author: dependabot[bot]

frontend/package.json

@@ -26,7 +26,7 @@
     "date-fns": "^4.1.0",
     "markdown-it": "^13.0.1",
     "mimetext": "^3.0.24",
-    "next": "14.2.13",
+    "next": "14.2.15",
     "nhs-notify-web-template-management-amplify": "*",
     "nhs-notify-web-template-management-utils": "*",
     "nhsuk-frontend": "^8.3.0",