Move duo pro trials controller to gitlab_subscriptions namespace

What does this MR do and why?

Move duo pro trials controller to gitlab_subscriptions namespace

• Moving to get closer to consolidating on gitlab_subscriptions when referring to subscriptions in the app.
• move controller, routes and affected tests to follow some convention and increase findability.
• see #461873

MR acceptance checklist

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

Screenshots or screen recordings

no change. See below. Note, if you do not see the background image(sparkles), there seems to be an issue with Vite config(issue needed) and ability to use background images this way and you can merely disable Vite if you have a desire to manually verify the sparkles like I did.

How to set up and validate locally

Note: verified by ee/spec/features/gitlab_subscriptions/trials/duo_pro/saas/* tests.

1. Setup GDK to simulate SaaS.
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. Visit `/-/trials/duo_pro/new

Related to #461873