Add confidential attr to API for notes update

What does this MR do?

It adds confidential param to notes update endpoint.

This also means we can set note param optional. This, however, needs to be covered in the note update service and in case the note param is empty we need to skip it (note does not allow having empty text anyway)

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.

Related issue: #207472 (closed)