VIA-684 AJ Bump next from 15.5.7 to 15.5.9 to fix high severity CVEs

- https://react.dev/blog/2025/12/11/denial-of-service-and-source-code-exposure-in-react-server-components

Author: ankur-jain-nhs

package-lock.json

@@ -77,7 +77,7 @@
     "isomorphic-dompurify": "^2.30.1",
     "jsonwebtoken": "^9.0.2",
     "jwt-decode": "^4.0.0",
-    "next": "15.5.7",
+    "next": "15.5.9",
     "next-auth": "^5.0.0-beta.29",
     "nhsuk-react-components": "^5.0.0",
     "openid-client": "^6.8.1",