Add modal window for duo pro purchase

What does this MR do and why?

Solves https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/8107

Remove check that hides "Add seats" button for duo pro and add modal with info on how to purchase.

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

w/ ramp	w/ reseller

How to set up and validate locally

Note: Customers App needs to be setup.

1. Purchase a ramp subscription with code suggestions.
2. Create a new group (via gitlab.com) and link the subscription to it.
3. Visit http://localhost:3000/groups/<NEW_GROUP_NAME>/-/usage_quotas#code-suggestions-usage-tab
4. Click "Add seats" button to confirm modal appears.
5. Repeat the same steps above for a reseller subscription.

Related to #8107 (moved)