VIA-667 AJ/AS Bump next from 15.5.6 to 15.5.7

- critical security fix from https://digital.nhs.uk/cyber-alerts/2025/cc-4723/cc-4723

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": "15.5.6",
+    "next": "15.5.7",
     "next-auth": "^5.0.0-beta.29",
     "nhsuk-react-components": "^5.0.0",
     "openid-client": "^6.8.1",