Use unit_price from invoice preview API response for price per year

What does this MR do and why?

Uses unitPrice from invoice preview API response for price per user per year value. When not using invoice preview API (when useInvoicePreviewApiInSaasPurchase feature flag is disabled), corresponding value from availablePlans is used (as before).

Screenshots or screen recordings

Screenshots can be found here.

How to set up and validate locally

Instructions can be found here.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #407599