SERVER-63498 made InternalTransactionNotSupported a retryable write error to allow a retryable WCOS update or findAndModify statement that fails to execute using an internal transaction during downgrade to be retried by drivers; the retry would use the legacy way of handling WCOS errors which doesn't require an internal transaction. Since every retryable write error is a transient transaction error, InternalTransactionNotSupported is now being returned with a TransactionTransactionError label, and that causes the transaction API to retry every internal transaction that runs during downgrade kTxnRetryLimit times before returning an error to the external client.