Leave current contacted_at value after unregistering last runner manager
When a user runs gitlab-runner unregister -u https://staging.gitlab.com -t glrt-some-token
to unregister an online runner, the runner becomes offline, thanks to the changes in #436475 (closed). However, the contacted_at
value is reported as null. We should ensure that the contacted_at
value is preserved, so that it is clear that this runner has had runner managers connected to it.