Merge branch '388075-use-graphql-api-to-preview-transaction-in-saas-purchase-with-ff' into 'master'

Use invoice preview API to calculate plan pricing for SaaS purchase

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/110835



Merged-by: Paul Slaughter <pslaughter@gitlab.com>
Approved-by: Sharmad Nachnolkar <snachnolkar@gitlab.com>
Approved-by: Josianne Hyson <jhyson@gitlab.com>
Approved-by: Paul Slaughter <pslaughter@gitlab.com>
Reviewed-by: Paul Slaughter <pslaughter@gitlab.com>
Reviewed-by: Josianne Hyson <jhyson@gitlab.com>
Reviewed-by: Wu Jeremy <jeremyw@jihulab.com>
Reviewed-by: Diana Zubova <dzubova@gitlab.com>
Reviewed-by: Sharmad Nachnolkar <snachnolkar@gitlab.com>
Co-authored-by: Vamsi Vempati <vvempati@gitlab.com>