Replace GlCollapsibleListbox with RefSelector in app/assets/javascripts/pipeline_new/components/refs_dropdown.vue

The GlDropdown in app/assets/javascripts/pipeline_new/components/refs_dropdown.vue was migrated to a GlCollapsibleListbox for #384288 (closed).

It may be that this could be further refactored to use the RefSelector component. So the goals of this issue are:

• Determine whether RefSelector is appropriate here
• Implement it if so, close this issue if not with details as to why

See &8528 for further details.