Api confidential issues

What does this MR do?

This MR adds support for confidential issues the the API. The property as exposed and users can create or update confidential issues.

What are the relevant issue numbers?

Closes #17199 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together