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
- ignore gitleaks finding that searches the whole repository

Author: ankur-jain-nhs

.gitleaksignore

@@ -8,3 +8,4 @@ src/app/_components/nhs-app/CardLink.tsx:ipv4:30
 4042f9b1238dd4efc8bfdee65fa4ecbd5e08504e:fake-api/data/apim/token.json:generic-api-key:6
 src/app/api/fake-login/token/route.ts:jwt:11
 src/app/_components/nhs-app/CardLinkWithDescription.tsx:ipv4:30
+1129d40a926e34bf9425dbe7064c59ea0e6742ca:src/app/_components/nhs-app/CardLinkWithDescription.tsx:ipv4:29