Enable `NOT` negation for Vue MR list Milestone filter

What does this MR do and why?

This is a follow-up to the caveats section of !153925 (merged).

We have a list of negatable filters, so we can now enable != for Milestone filtering.

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 options
Filtering to NOT v0.0

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 the != (NOT) exclusion
6. Select one of your milestones that would exclude some MRs
7. 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