Add migration to cleanup broken epic-issue links

Follow-up to #38281 (closed)

The security MR addressed the problem with project transfer where issues that have an epic assigned to them would retain the link even after the project was transferred to a different group.

For previous transfers with existing broken links, we need a migration that checks if the issue's epic belongs to the same group as the issue's project (or an ancestor of this group), and if it doesn't, the link should be destroyed.