fix the import :milestone from adding the group_id
What does this MR do?
This fix an import validation error.
Why was this MR needed?
Importing a project that contains issues and milestones was failing with a validation error.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- Documentation created/updated
- API support added
- Tests added for this feature/bug
- Has been reviewed by UX
- Has been reviewed by Frontend
- Has been reviewed by Backend
- Has been reviewed by Database
- Conform by the merge request performance guides
- Conform by the style guides
- Squashed related commits together
What are the relevant issue numbers?
Closes #35580 (closed)