In laravel 12 you can no longer use setBody instead you should use mailable classes in notifications. You also need to return a Mailable instead of a MailMessage, you will need to specify the message recipient using the mailable object's to method
php artisan make:mail YourMailableClass
public function toMail($notifiable): Mailable
{
$mail = new YourMailableClass;
return $mail->to($notifiable->email)->html($this->html);
}