Documentation for PHPUnit  is out of date

Page at https://docs.codewars.com/languages/php/phpunit

According to https://github.com/codewars/content-issues/wiki/List-of-PHP-Kata-to-Update,  from code runner version `PHP 8.0` on, the solution should now start with the tag `<?php` ;and the tests should have:

```php
<?php

use PHPUnit\Framework\TestCase;

class Test {
// ...
```