Work Item Quick Action: Set confidentiality
# Summary
Work Items support confidentiality starting %"15.8" via https://gitlab.com/groups/gitlab-org/-/epics/9547. We need to add support for `/confidential` quick action to mark a Work Item as confidential, additionally, it should also take into account the following restriction before applying confidentiality; A work item can only be marked as confidential if; a) It doesn't have any child work item or b) All of its child work items are already confidential.
issue