Use ListBox to improve country/state selection for in-app trials

What does this MR do and why?

• for in-app trials
• make sorting/searching easier and also fulfill the needs of https://gitlab.com/gitlab-org/gitlab/-/issues/461120. See #549546 (comment 2568719865)
• use listbox instead here for a more desirable UX component.
• see #549546 (closed)

Screenshots or screen recordings

item	Before	After
country
state

How to set up and validate locally

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
2. In rails console enable the feature flag

   Feature.enable(:ultimate_trial_single_form)

3. visit /-/trials/new path.

MR acceptance checklist

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Related to #549546 (closed)