First iteration for making duo enterprise in-app trials 1 step

What does this MR do and why?

First iteration for making duo enterprise in-app trials 1 step

• break up the work into multiple iterations
• working towards the goal of making the trial submission 1 step instead of 2.
• introduce new in-app trial form for new only.
• move names of existing over to legacy_ when necessary as those old 2 step code paths will be going away once rollout is complete.
• see prior work for ultimate trials at /-/trials/new
• see https://gitlab.com/gitlab-org/gitlab/-/issues/543764

Screenshots or screen recordings

Click to expand

page	Before	After
/-/trials/duo_enterprise/new
/-/trials/duo_enterprise/new mobile
/-/trials/duo_enterprise/new video	Screen_Recording_2025-08-05_at_12.57.36_PM	Screen_Recording_2025-08-05_at_12.52.03_PM

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. Create new group.

3. In rails console make it ultimate.

   bin/rails c
   
   Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'ultimate'))

4. In rails console enable the feature flag

   Feature.enable(:duo_enterprise_trial_single_form)

5. visit /-/trials/duo_enterprise/new path.

MR acceptance checklist

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Related to #543764