Change plan card styling for trial billing page

What does this MR do and why?

Change plan card styling for trial billing page

• styling around header and cta for purchase changes
• add duo core feature behind feature flag
• see https://gitlab.com/gitlab-org/gitlab/-/issues/534708

Changelog: other EE: true

Screenshots or screen recordings

plan	Before	After
free
ultimate trial

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. Enable reveal_duo_core_feature feature flag in rails console(bundle exec rails c).

   Feature.enable(:reveal_duo_core_feature)

Free namespace

1. Create a new top-level group
2. visit the billings page for that group

Ultimate trial namespace

1. Create a new top-level group
2. In the gitlab admin UI, edit the group to be on an ultimate trial plan(edit button from http://gdk.test:3000/admin/groups)
3. In rails console(bundle exec rails c), change the trial, trial start and end date for the group

   Group.last.gitlab_subscription.update_columns(trial_starts_on: Date.today, trial_ends_on: 30.days.from_now, trial: true)

4. visit the billings page for that 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 #534708