[Checkout Branding API] Deprecation of transparent background

We're deprecating the TRANSPARENT value on the CheckoutBrandingBackground enum of the Admin GraphQL API checkoutBranding query and checkoutBrandingUpsert mutation. Use BASE or SUBDUED instead.

We will remove this value from the mutation input and the GraphQL API in the subsequent API version (2024-07). To prepare for the 2024-07 API version release, stop using the TRANSPARENT value.