Resolve "Issue boards sidebar milestone dropdown should not be multi select"

What does this MR do?

• Prevent the Milestone dropdown from allowing multiple items being selected at once
• Fix removing the milestone if unchecked

Why was this MR needed?

• User could select multiple milestones and an issuable can only belong to one milestone

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #31474 (closed)