Set the price of a variable product to the minimum price of its variations on Product Feed

[nima-karimi]

The price of the parent variable product must be set to the minimum price of its variations/children. This is how the tracking event is set up, so we should change the feed to be consistent with that:

https://github.com/woocommerce/facebook-for-woocommerce/blob/28ccf99ff0cb67e01a9651b73058e2386cfb16b1/facebook-commerce-events-tracker.php#L509-L513

We also apply the same idea to other similar product types:
https://github.com/woocommerce/facebook-for-woocommerce/blob/c034913a6113af1101e8fb1af83f37ad6a66ba0e/includes/Products.php#L334-L347