[Php81] Skip ReadOnlyPropertyRector on property changed via reference in foreach

[TomasVotruba]

Skip making a property readonly when its array is mutated through a by-reference foreach (foreach ($this->items as &$item)).

Such a property is still being written to after its initial assignment, so marking it readonly would cause a "Cannot modify readonly property" runtime error.

final class SomeClass
{
    private array $relativeDateStrings;

    public function __construct()
    {
        $this->relativeDateStrings = ['yesterday', 'today'];
        foreach ($this->relativeDateStrings as &$string) {
            $string = strtoupper($string);
        }
    }
}