Add milestone filter to Vue merge request list app

What does this MR do and why?

For #442917 (closed)

Adds the MR milestone filter to the Vue MR list search bar.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

-	Before	After
Filter option dropdown
Filtered results	x

How to set up and validate locally

1. Enable the vue_merge_request_list feature flag in the rails console rails c => Feature.enable( :vue_merge_request_list )
2. Make sure that at least 1 MR has a milestone assigned (MRs with different milestones would be useful)
3. Navigate to the Merge Request list e.g. http://gdk.test:3443/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing milestone in the search bar, and select the milestone filter
5. Select one of your milestones that would include / exclude some MRs
6. Submit the search / filter
   1. Try refreshing the page completely to test the URL search parameter on initial load
   2. Try clearing the milestone token filter and submitting that to see all the MRs become visible

Caveats

1. We're not currently able to connect the negated milestone filter to the resolver, as we don't have a list of negatable filters
   • Due to this, the not operator is removed from this implementation until the back end fully supports it
2. The merge requests list backend does not yet support wildcard filters
   • We have agreed to ship this without wildcards, since it's feature-flagged anyway.