Fix Configurable Product Type resource model 'getChildrenIds' method …

[VitaliyBoyko]

…return format

Has been fixed return type of Magento\ConfigurableProduct\Model\ResourceModel\Product\Type\Configurable:getChildrenIds method.

Fixed Issues (if relevant)

1. Configurable Product Type resource model 'getChildrenIds' method incorrect return format. #14061: Configurable Product Type resource model 'getChildrenIds' method incorrect return format.

Manual testing scenarios

1. Inject class \Magento\ConfigurableProduct\Model\ResourceModel\Product\Type
2. Apply the array of multiple configurable ids to method getChildrenIds
3. Expecting array to have such structure: [
   "parent id" => ["child id"]
   ]

[magento-cicd2]

All committers have signed the CLA.

[orlangur]

Isn't this a backward-incompatible change for anyone who used this method before? Two-dimensional array is changed to one-dimensional.

Why 2.3-develop is targeted instead of 2.2-develop?

[VitaliyBoyko]

Hi @orlangur!
Thank you for review.
I will make backports for other tags when this one will be accepted.