Finalize backfilling of due dates from epic to work item

What does this MR do and why?

Finalize BackfillEpicDatesToWorkItemDatesSources migration. It got enqueued the first time by !153115 (merged)

Related to: #454427 (closed)

The migration was confirmed as finished in https://gitlab.slack.com/archives/C101F3796/p1717755963067099

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