Fix Issue boards don't show milestone in issue details

What does this MR do?

Milestones were not being displayed in the issue board sidebar. Now use correct entity, API::Entities::Milestone

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/55301

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• Security reports checked/validated by reviewer