Fix duplicate design comments being created during issue move

What does this MR do?

This MR fixes a bug where comments on designs would be duplicated during an issue move, but only for designs that had more than one comment.

It does not have a changelog entry because the feature has not been enabled on production yet.

• Issue: #13426 (closed)

QA Steps

1. Enable design_management_copy_designs feature: Feature.enable(:design_management_copy_designs)
2. Add a design to an issue (see docs)
3. Add more than one comment to a design (see docs)
4. Move the issue (the button is on the issue sidebar)
5. Reload until the designs appear (the copy happens on a worker queue)
6. 🎉 there should be the same number of comments on designs as there was before the move

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done