Migrate base product validations to the new validation methods

Objective

See the objective outlined in the parent epic [Unify purchase flows] SaaS/Self-Managed Premiu... (&18567).

Purpose

This issue is to migrate base product validations for SaaS and Self-Managed subscriptions to the new validation methods:

• Namespace should be present for Saas purchase
• We should allow expired reseller subscriptions to purchase We don't need to worry about this, based on conversation: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/15252#note_3200938296
• The existing subscription can't have duo self-hosted ✅