Fix moving epics to a new group

What does this MR do and why?

Fix moving epics to a new group

When moving an epic to another group, we copy all the related data over to a new epic, or rather its related work item.

As part of the epic work item migration, every epic has a correlating work item (issue_id), and for some shared associations between the epics and the issues table, we still use the old epic_id OR the new issue_id.

Usually this is handled via a type column, like noteable_type and notable_id. In the case of description_versions we either set the epic_id or the issue_id in the column, and have a XOR NOT NULL constraint.

So there are still many description_version records with an epic_id, and when we copy over the data as part of moving the epic, we copied over the epic_id AND the new work item issue_id. This resulted in a database constraint check error, since we can't set both values at once.

This change fixes the copy mechanism by removing the epic_id when copying the description_version attributes. The epic_id is no longer needed, as we can associate it anyway with the new work item (issue_id).

Changelog: fixed EE: true

References

Issue: https://gitlab.com/gitlab-com/request-for-help/-/issues/3318 Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1846374/?alert_rule_id=52&alert_type=issue&environment=gprd&notification_uuid=cd5ea1b4-9daf-4549-8653-89efa217cf67&project=3&referrer=slack

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create an epic
2. Change the description to generate a description versions (notable in the "Description changed" system note)
3. Then modify the epic record to reproduce the state:

   dv = DescriptionVersion.last
   dv.update!(issue_id: nil, epic_id: Epic.find_by_issue_id(dv.issue_id).id)

4. Move the epic to another group like /move other-path
5. Before this MR: you see a 500 error, after this MR the move will be successful

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