Hide duo pro for premium tier with enterprise add-on

What does this MR do and why?

A Duo Enterprise customer should not see the option to start a Duo Pro trial (whether they are Premium or Ultimate)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Create a new group.

bin/rails c

Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'premium'))

GitlabSubscriptions::AddOnPurchase.create(subscription_add_on_id: GitlabSubscriptions::AddOn.find_by(name: 'duo_enterprise').id, namespace: Group.last, started_at: 0.days.ago, expires_on: 1160.day.from_now, trial: false, quantity: 100, purchase_xid: '1', organization_id: 1
)

• Visit group's billing page.

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 #552355