[DowngradePhp70][Transform] Add #[\ReturnTypeWillChange] on Downgrade + transform ArrayObject::getIterator() to keep working on php 8.1

[samsonasik]

Given the following code, on DowngradeScalarTypeDeclarationRector:

class ArrayStack extends \ArrayObject
{
    public function getIterator(): \Iterator
    {
        $array = $this->getArrayCopy();
        return new ArrayIterator(array_reverse($array));
    }
}

It only remove \Iterator and add @return \Iterator, and if the code is using in php 8.1, it will got notice:

Deprecated: Return type of ArrayStack::getIterator() should either be compatible with ArrayObject::getIterator(): Iterator, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /in/uZl8U on line 8

ref https://3v4l.org/uZl8U#v8.1.2

This PR fix it by register it to PhpDocFromTypeDeclarationDecorator::ADD_RETURN_TYPE_WILL_CHANGE constant value.

ref laminas/laminas-stdlib#54 for inspiration.

[samsonasik]

Tested in 2 rules:

Rector\DowngradePhp70\Rector\FunctionLike\DowngradeScalarTypeDeclarationRector;
Rector\Transform\Rector\ClassMethod\ReturnTypeWillChangeRector;

[samsonasik]

All checks have passed 🎉 @TomasVotruba it is ready for review.