Ensure that requirement belongs to same project of work item

What does this MR do and why?

Add model validation not allowing requirements to belong to a different project from its work item.

Technical debt described at #377474 (closed)

There is no indication that this is happening in production or self-managed instances, but I think it is still worth having this validation at least at the model level as we cannot remove project_id column from requirements table.

Production check

gitlabhq_production=> SELECT COUNT(*) FROM requirements 
gitlabhq_production->   INNER JOIN issues ON issues.id = requirements.issue_id
gitlabhq_production->   WHERE issues.project_id != requirements.project_id \g
 count 
-------
     0
(1 row)

MR acceptance checklist

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