Resolve "Refactor Confidentiality sidebar component to use Vue +Apollo"

What does this MR do?

Currently, issue sidebar Confidentiality component is implemented with the binding to notes app Vuex store. In order to align all sidebar components to use the same stack, make them reusable and be ready for GraphQL subscription-based real-time updates, we refactor this component to use Vue + Apollo client stack with no Vuex bindings.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style 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

Related to #300654 (closed)