Fix all core processors to implement doPayment and not doTransferPayment or doDirectPayment
Summary from @KarinG
doDirectPayment and doTransferCheckout will soon be flagged deprecated PR in Core: https://github.com/civicrm/civicrm-core/pull/17456
Here's a documentation block: https://github.com/civicrm/civicrm-core/blob/master/CRM/Core/Payment.php#L1324-L1332
If your Payment Processor code is using doDirectPayment -> here's how to move to doPayment!
1a -> note that doPayment should throw an exception rather than return an error: Specific Example by @eileen -> replacing a CRM_Core_Error with a throw new PaymentProcessorException https://github.com/civicrm/civicrm-core/pull/17565/files#diff-abf3be8fd9a6ed43b68f6d8439c85f2fL140
1b -> then rename doDirectPayment -> doPayment Example PR by @KarinG https://github.com/iATSPayments/com.iatspayments.civicrm/pull/310
1c -> testing! Don't forgot to also test an unsuccessful transaction -> then you know what your notices will look like.
1d -> if you have any doTransferCheckout -> replace that with doPayment as well.