Show `!=` NOT filter for target branch in the Vue MR List

What does this MR do and why?

When I was working on porting the Target Branch filter option to the new Vue MR list page, I didn't initially realize I needed to raise the issue that the implementation didn't support the != filter.

This adds the != filter option for the MR target branch filter.

References

• Initial issue to add the Vue filter: #442923 (closed)
• Implementation: !150757 (merged)
• Adding support for != to the BE: !170664 (merged)

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 Bar
Actual != filtering	You can't use != Target Branch filtering with the feature flag enabled. Here's the list page with an MR that has a non-master target branch.

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 you have at least 1 MR (it would probably be helpful to have an MR that targets a branch other than master, although this isn't required)
3. Navigate to the Merge Request list e.g. http://gdk.test:3443/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing target branch in the search bar, and select the Target Branch filter
5. Select the != (NOT) exclusion
6. Select one of your target branches that would exclude at least one MR
7. Submit the search / filter
   1. Try refreshing the page completely to test the URL search parameter on initial load
   2. Try clearing the target branch token filter and submitting that to see all the MRs become visible