Accept any instance of Rule and not just Password (#37407)

This would be useful if there a requirement to use custom Password implementation such as `Laravel\Fortify\Rules\Password` etc.

src/Illuminate/Validation/Rules/Password.php

@@ -123,7 +123,7 @@ public static function default()
     {
         $password = value(static::$defaultCallback);
 
-        return $password instanceof static ? $password : static::min(8);
+        return $password instanceof Rule ? $password : static::min(8);
     }
 
     /**