Create audit log event on every code path where a runner is deleted

As seen in https://gitlab.com/gitlab-com/ops-sub-department/section-ops-request-for-help/-/issues/430, we have scenarios where a runner is mysteriously deleted, and we have no idea what happened, and no means of investigating since all data was removed with the runner. We should audit every code path leading to a runner being deleted, such as Ci::Runners::BulkDeleteRunnersService, stale runner deletion, etc.