Increase registry GC overdue task queue alert time to account for instance worker recruitment

Context

We're getting mostly false alarms from this alert, apparently due to bursts of deletes from user activity. Rather than increasing the threshold, I believe we should increase the window of time to account for sleeping GC workers to wake up. This will help alert us only when the queues are failing to empty over prolonged periods of time, rather than on spikes of activity such as the following: