Auto Discount Eligibility - don't allow subsequent discounts

Problem

From https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7471#note_1455996384

We currently do not have an explicit check that prevents a subscription with a discount from receiving a 2nd discount. Instead we rely on the implicit behavior from the discounting window. This should cover all cases but I think adding an explicit business rule will help from a maintainability/self documentation viewpoint.

Proposal

Add a check to AutoRenew::AutoDiscountEligibilityService to error if the subscription already has a discount.

Result

Next steps (if any)

How will we measure success?