NOT NULL constraint for Epic Issues work_item_parent_link_id

What does this MR do and why?

Now that all EpicIssue records have been backfilled with the correct work item parent link ID, we can add a DB constraint to ensure that the EpicIssue never has a NULL fk reference to the WorkItemParentLink that models that same relationship

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Before	After

How to set up and validate locally

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Related to #504270 (closed)