Call to undefined method Cake\Mailer\AbstractTransport::_headersToString()

Upgrading CakePHP3 to CakePHP4, getting this error:

Call to undefined method Cake\Mailer\AbstractTransport::_headersToString()

The method was moved to src/Mailer/Message.php in ce8c7e8. Instead, call getHeadersString() providing an array of headers you need.

$headerNames = [
    'bcc',
    'cc',
    'from',
    'readReceipt',
    'replyTo',
    'returnPath',
    'sender',
    'subject',
    'to',
];
// get all the headers
$headers = $email->getHeadersString($headerNames);