[5.4] Add salutation option to SimpleMessage notification

[seblavoie]

By default "Regards," is displayed as a salutation at the end of notification emails. Much like the pull #15108 added an optional greeting() method, I think that would make sense to also add one for the salutation at the end. What do you think?

public function toMail($notifiable)
{
    return (new MailMessage)
                ->line('The introduction to the notification.')
                ->action('Notification Action', 'https://laravel.com')
                ->line('Thank you for using our application!');
                ->salutation('Cheers!')
}

This was originally proposed on the 5.3 branch (#17422).