Remove upper bound on sent_notifications backfill

What does this MR do and why?

In QueueBackfillSentNotificationsAfterPartition migration we set a specific start_id for .com, so we can skip backfilling billions of records. This might cause problems with local GDKs and also environments where Gitlab.com_except_jh? might return true. So, we also need to make sure the record exists in .com before scheduling the migration

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