Relate quick actions doesn't sync back to legacy epics

Bug description

When using the /relate quick action on epic work items, we don't sync the data back to the legacy epic.

How to reproduce

1. Go to an epic
2. Create a comment with /relate #OTHER_EPIC_IID
3. Add ?force_legacy_view=true to the URL
4. The related epic is not shown

The problem

In the InterpreterService, we use the IssueLinks::CreateService, not the WorkItems::CreateService. Therefore the code to create the synced legacy epic data is never called.

The /unlink action has the same problem, but due to the FK constraint, the related_epic_links record will be deleted anyway.

This got introduced in %17.7 with !172901 (merged).

Impact

On GitLab.com it's just 4 related epics that are mismatched. There is no visible user impact, as they still see the work item version. However:

• Epic-Epic relations that are linked via /relate don't have the legacy record
• When using the REST or GraphQL Epic API, the linked items would not be shown

How to fix

1. ✅ Use the proper service
2. Backfill the data OR fetch the data from work items