Convert DuoPro trials selection step form to view component

What does this MR do and why?

• Improved encapsulation and maintainability through isolated testing
• Remove view logic from controller layer
• Covered by unit/integration tests and tests in ee/spec/support/shared_examples/components/duo_pro_trial_form_shared_examples.rb

References

• !183257 (merged) - Convert DuoEnterprise trials selection step form to view component
• Merged DuoEnterprise Lead Form Component MR
• Similar Related MR

Screenshots or screen recordings

No UI changes.

Before	After

How to set up and validate locally

1. Setup GDK to simulate SaaS.

2. Create a new group

3. Edit the group subscription in the rails console to be on an Premium plan (per ee/app/models/gitlab_subscriptions/duo_pro.rb) or as a shortcut you can edit this in DuoCommon

   def check_trial_eligibility!

   -        return if eligible_for_trial?

   +        return true

   +        # return if eligible_for_trial?

4. Visit /-/trials/duo_pro/new?step=trial

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #517154 (closed)