Resolve "Projects hooks : confidential_issues_events"

What does this MR do?

It aligns the project hook "confidential_issues_events" implementation with the "issues_events" one

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Until this MR, one can't use the API to fully manage the projects hooks. One have to use the user interface in order to enable/disable the confidential issues events trigger. (See #43425 (closed))

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#43425 (closed)

Closes #43425 (closed)