Update generated code

[stripe-openapi[bot]]

Codegen for openapi v1268.

Changelog

• Add support for new resources Billing.CreditBalanceSummary, Billing.CreditBalanceTransaction, Billing.CreditGrant, and Margin
• Add support for retrieve method on resource CreditBalanceSummary
• Add support for list and retrieve methods on resource CreditBalanceTransaction
• Add support for create, expire, list, retrieve, update, and void_grant methods on resource CreditGrant