Resolve "Increase jitter for AdjournedGroupDeletionWorker"

What does this MR do and why?

Related to #346170 (comment 824957958)

Based on the graph here, the 95 percentile of GroupDestroyWorker originating from AdjournedGroupDeletionWorker is close to 20 seconds.

We currently have a jitter of only 10 seconds, which is close to the 75th percentile number. So, if we can increase the jitter between groups from 10 seconds to 20 seconds, we can ensure that more jobs finish processing before the next one is enqueued, which in turn prevents crowding of jobs which is possibly currently resulting in some ActiveRecord::QueryCanceled errors while these jobs are run.

This MR also changes the start time of the AdjournedGroupDeletionWorker cron to 2 AM everyday. With !81521 (merged), AdjournedProjectsDeletionCronWorker now runs at 7 AM everyday, so here we have a comfortable 5 hour gap between the two to ensure that jobs from these 2 cron workers do not overlap.

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Related to #346170 (closed)