Add active state to milestones fetch in filtered search

What does this MR do and why?

An alumni reported they were seeing closed milestones in the new filtered search dropdown.

This MR filters board milestones by active.

Repro

• rails c
• Feature.enable(:issue_boards_filtered_search)
• Go to Project or Group Issue Boards
• Select the milestones token, then = or != then notice that the milestones list only contains active milestones

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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