Update the schema stub to use iterable for the fields and filters

[shanginn]

In the core/src/Core/Schema/Schema.php both fields and filters returns iterable, but stubs still generate them as arrays:

abstract public function fields(): iterable;

public function filters(): iterable

[lindyhopchris]

Thanks for the PR. I'm not sure it needs to be iterable as the generated schema is returning an array, and array is allowed as the return type as it is a covariant.

Do you have a specific reason for changing this? I.e. are you not returning array from those methods?

[shanginn]

yeah, you are right, array is a covariant of iterable. all I want to do is make PHPStan happy with the default generated schemas.
how about this then? Just add iterable type to the phpdoc?