Fix typehint for ResetPassword::toMailUsing()

[KKSzymanowski]

The type hint the $callback parameter in ResetPassword::toMailUsing() seems to be too narrow - it declares that the Closure must return an instance of MailMessage and nothing else. However an instance of Mailable is perfectly acceptable in this case as it goes through MailChannel::send() which has a special case for $message instanceof Mailable.

This triggers the following error eg. in phpstan:

Parameter #1 $callback of static method Illuminate\Auth\Notifications\ResetPassword::toMailUsing() expects Closure(mixed, string): Illuminate\Notifications\Messages\MailMessage, Closure(mixed, mixed): App\Mail\ResetPassword given.

I believe the type hint should be corrected to include Mailable as an acceptable Closure return type.