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

What does this MR do and why?

When I was working on porting the Source 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 source branch filter.

References

• Initial issue to add the Vue filter: #442924 (closed)
• Implementation: !150758 (merged)
• Adding support for != to the BE: !170663 (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 != Source Branch filtering with the feature flag enabled. Here's a filter showing which MR has the signed-commits source 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 (more than 1 would be useful, to ensure the correct results are being returned, and not just empty results)
3. Navigate to the Merge Request list e.g. http://gdk.test:3443/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing source branch in the search bar, and select the Source Branch filter
5. Select the != (NOT) exclusion
6. Select one of your source 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 source branch token filter and submitting that to see all the MRs become visible