Use invoice preview API to calculate plan pricing for SaaS purchase

Follow upstream code changed. The merge code is https://gitlab.com/gitlab-org/gitlab/-/merge_requests/118710#note_1367848986

Detail

## Problem

when remove ff use_invoice_preview_api_in_saas_purchase

http://gdk.test:3000/-/subscriptions/new?namespace_id=180&plan_id=2c92c0f876e0f4cc0176e176a08f1b70

In customersdot, the graphql InvoicePreview not avaliable for this condition. from upstream branch jihu-2023-04-04 https://gitlab.com/gitlab-jh/customers-gitlab-cn/-/blob/jihu-2023-04-04/app/graphql/resolvers/invoice_preview.rb

Can we fetch the customerdot last code from upstream?

/cc @dexterdeng