Automatic user deactivation isn't logged to Audit events

Summary

In 16.0 we added user deactivate service that logs user deactivation events to Audit events.

However, we do not use this service for automatic dormant user deactivation which leads to these events not getting logged to audit events.

Steps to reproduce

1. Make sure have a user that is dormant on your GitLab instance.
2. Enable Automatic deactivation of dormant users.
3. Run Users::DeactivateDormantUsersWorker.new.perform worker through Rails console.
4. The dormant user should get deactivated, but you won't see the deactivation event logged to Audit events

Example Project

Not reproducible on GitLab.com

What is the current bug behavior?

No log of user deactivation

What is the expected correct behavior?

We should log user deactivation when it is done automatically as well.

Possible fixes

Use Users::DeactivateService service in automatic dormant user deactivation worker.

cc @s_shaik