fix: update vulnerable dependencies

- lxml: ^5.1.0 -> ^6.1.0 (CVE fix, earliest fixed version 6.1.0)
- pytest: ^8.4.2 -> ^9.0.3 (CVE fix, earliest fixed version 9.0.3)
- werkzeug: add >=3.1.6 (CVE fix, earliest fixed version 3.1.6)
- openapi-core: ^0.19.5 -> ^0.20.0 (unblock werkzeug update)
- pytest-nhsd-apim: ^5.0.6 -> ^6.0.0 (unblock lxml and pytest updates)

Author: EdwardWills-nhs