Feature flag to rollout new CustomersDot Staging URL

What does this MR do and why?

This MR introduces a new FF that will help to easily switch from the current CustomersDot Staging url (customers.stg.gitlab.com) to the new one (customers.staging.gitlab.com).

Related issue: https://gitlab.com/gitlab-com/gl-infra/customersdot-ansible-poc/-/issues/54

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