milestone_select: fix changing the milestone from the sidebar

What does this MR do?

milestone_select: fix changing the milestone from the sidebar

5d0a829d introduced a regression that prevented changing the milestone for an issue in the sidebar on the issue board. An import was added with the same identifier as a locally scoped variable. This changes the locally scoped variable identifier.

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/54139

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54139

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.