Add information about note confidentiality to APIs

What does this MR do?

Add information about note confidentiality to

• NoteType for graphQL
• notes end points in public API
• notes in Rails app

It is not possible to set confidetial notes, we don't have this feature done yet therefor I didn't create a changelog entry.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Backend for #207471 (closed)