[Form] Fix "Array was modified outside object" in ResizeFormListener.

The onSubmit() method of the ResizeFormListener class is assuming the data is an array, and calling unset directly inside a foreach. This works fine in most scenarios, but if data is an instance of IteratorAggregate that is iterating over $data by reference, it breaks with the following error:

Symfony\Component\Form\Extension\Core\EventListener\ResizeFormListener::onSubmit(): ArrayIterator::next(): Array was modified outside object and internal position is no longer valid in ./vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Core/EventListener/ResizeFormListener.php line 142

This is because the foreach loop is using an Iterator in the background, but the ResizeFormListener has unset the underlying data directly, causing the value that the Iterators internal pointer is pointing to to change.

The Iterator provided by IteratorAggregate may or may not have the ability to modify it's underlying data. So it is not possible to rely on the Iterator to modify the data. Instead, it is simpler to avoid modifying $data at all while we are iterating over it. So instead, we simply iterate over $data once to determine the keys we need to delete, and store them. Then we iterate over the keys and delete them from $data.

Fixes #10405