rector/config/set/swiftmailer60.php

<?php

declare(strict_types=1);

use Rector\Renaming\Rector\Class_\RenameClassRector;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;

return static function (ContainerConfigurator $containerConfigurator): void {
    $services = $containerConfigurator->services();

    $services->set(RenameClassRector::class)
        ->call('configure', [[
            '$oldToNewClasses' => [
                # swiftmailer 60
                'Swift_Mime_Message' => 'Swift_Mime_SimpleMessage',
            ],
        ]]);
};