Update merge request assignee dropdown to GlCollapsibleListbox

What does this MR do and why?

Updates the assignee dropdown on merge requests to use the GitLab UI GlCollapsibleListbox to match the other dropdowns in the sidebar.

The assignee sidebar was the last merge request dropdown still on the deprecated GlDropdown by way of user_select.vue. It now renders a new AssigneeDropdown component, opened by an Edit button in the title row, in the same shape as the reviewer, milestone and label dropdowns.

Screenshots or screen recordings

How to set up and validate locally

1. Visit a merge request in a project where you can edit it.
2. Click Edit next to Assignee in the right sidebar, or press a.
3. Assign and unassign users, and confirm the sidebar list updates.

MR acceptance checklist

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