[Experiment] Position Premium Trial at Registration

What does this MR do and why?

This experiment tests different messaging approaches on the trial registration page to optimize conversion rates. The control variant shows Ultimate plan features with "full access to our best plan" messaging, while the candidate variant focuses on Premium plan features with "full access to Premium" messaging.

Location	Old Copy	New Copy
Sub-header	Enjoy 30 days of full access to our best plan, after which you'll have access to our free tier forever. Upgrade to the best plan that suits you at any point - Premium or Ultimate.	Enjoy 30 days of full access to Premium, after which you'll have access to our free tier forever.
Right Panel	One platform for Dev, Sec, and Ops teams End-to-end security and compliance Boost efficiency and collaboration Ship secure software faster GitLab Duo Enterprise: AI across the software development lifecycle	AI Chat in the IDE AI Code Suggestions in the IDE Release Controls Team Project Management Unlimited Licensed Users

References

https://gitlab.com/gitlab-org/gitlab/-/issues/562183

Screenshots or screen recordings

Control	Variant

How to set up and validate locally

1. Make sure Simulate SAAS is enabled
2. Open rails console
   1. rails c
   2. Feature.enable(:premium_trial_positioning)
3. Make sure you are logged out
4. Make sure customer dot is setup locally
   1. If not, you can mock CreateLeadService and return a successful response
5. Open /-/trial_registrations/new
6. You should see the candidate variant with updated copy
7. Complete the 6-step flow
8. Make sure assignment event is being triggered after creating your first group.

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 #562183