Correct parameter type of Collection::diffKeys() and Collection::diffKeysUsing()

[AJenbo]

Corrected the parameter type in the PHPDoc for Collection::diffKeys() and Collection::diffKeysUsing() from array<TKey, TValue> to array<TKey, mixed>, as these methods only operate on the keys of the given array, making the value type irrelevant.

This lets you do something like:

$animals = Animal::pluck('name', 'id');
$animalFood = Food::pluck('id', 'animal_id');

if ($animals->diffKeys($animalFood) as $id) {
    throw new Exception('Missing food for ' . $animals[$id]);
}

Tools previously flagged this usage as illegal due to the differing value types (food.id as int vs. animal.name as string), despite only the keys being compared.