When DELETING a user, warn Admin user is part of an on-call schedule

What does this MR do?

Adds a message to the confirmation modal that the user is part of the Oncall-schedule on deleting him/her

🇺🇦 you should have the feature flag enabled to see this message

 Feature.enable(:vue_admin_users)

Also, you should have at least GitLab Premium license and admin access

👀 To check

1. Navigate to Operations/On-call schedules or a direct link is your_project/-/oncall_schedules e.g. http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/oncall_schedules
2. Create a schedule https://docs.gitlab.com/ee/operations/incident_management/oncall_schedules.html#on-call-schedule-management

3. After you'd be able to create schedule rotations

4. Add some user to the rotation

5. Try deleting the user => got to Admin area/Users(http://127.0.0.1:3000/admin/users) => In the users' table in the context menu try deleting the user. You should see the message that user is a part of oncall-schedule in the confirmation modal.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this functionality is hidden behind the feature flag .
• 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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #323629 (closed)