Changes to Downgrade — release the N newest live mailboxes and lower the billed quantity.

added the new large_order_payment_pending enum value to the nextSteps/steps/items/reason response property for the response status 200

added the optional property billing/domainEnding to the response with the 200 status

added the optional property billing/prepay to the response with the 200 status

added the optional property nextSteps/steps/items/effect/oneOf[#/components/schemas/MailboxBilling]/domainEnding to the response with the 200 status

added the optional property nextSteps/steps/items/effect/oneOf[#/components/schemas/MailboxBilling]/prepay to the response with the 200 status