Update POST /checkout endpoint

๐Ÿšจ Deprecation Notice: The lightning_network parameter will be deprecated by 1 of June.
๐Ÿ”น Instead, please use the platform_id parameter for specifying Lightning Network payments.
๐Ÿ”น The Lightning platform ID can be found here
๐Ÿ”น Update your integration accordingly to avoid issues with future API versions.