Take confidential attr into account in NotePolicy

What does this MR do?

It starts taking confidential attribute into consideration in NotePolicy. Only project members and noteable authors or assignees can read confidential notes on the noteable.

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.

Issue: #209755