Fix duplicate boardsStore variable in milestone select

What does this MR do?

Rename one occurrence of boardsStore in milestone select class because it was shadowing another one.

What are the relevant issue numbers?

closes 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.
• Security reports checked/validated by reviewer