[CI Minutes] Add group eligibility criteria specifically for add on purchases

Problem

Currently the CI minutes purchase flow on GitLab.com is re-using the group eligibility logic for new subscription purchases. This means that any namespaces which already have an active subscription will not show up in the CI minute flow because they are not eligible for a new subscription.

Proposal

We should use a different eligibility criteria for this flow, as these groups should be eligible to purchase add-on packs.

This eligibility is currently being determined here: https://gitlab.com/gitlab-org/gitlab/-/blob/e266a52b/ee/app/controllers/subscriptions_controller.rb#L117