Added merge trains link to the Vue merge request list refactor

What does this MR do and why?

Added the merge trains link to the nav buttons in the Vue merge request list refactor

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

How to set up and validate locally

• Enable vue_merge_request_list feature flag
• Have merge trains enabled
• Visit /gitlab-org/gitlab-test/-/merge_requests/
• Merge trains link is visible in header