Account for new SaaS plans/SKUs

Slack discussion: https://gitlab.slack.com/archives/C01F9S37AKT/p1611894021164200 (90-day retention only)

We have new plans for the renamed SaaS offerings: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/staging/app/models/plan.rb#L310-321

I’m guessing we might still need to retain the bronze option for legacy reasons.