Only suggest confidential issues when adding issue to confidential epic

Summary

Public issues are suggested when the user attempts to add an existing issue to a confidential epic. It's not possible to actually add public issues to a confidential epic so these should be filtered out.

Steps to reproduce

1. Visit a confidential epic
2. Click 'Add' and select 'Add an existing epic'
3. Type a # the issue name field

Example Project

https://gitlab.com/groups/gitlab-org/epic-example/-/epics/8

What is the current bug behavior?

Public issues are suggested

What is the expected correct behavior?

Only confidential issues should be suggested

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)