Commit d9f9e74bd added a check to `WC_Product::set_tax_class()` to only accept valid tax classes, but this created a bug for product variations as this type of product has an extra tax class called 'parent'.
This commit fixes this problem by adding a new method to `WC_Product` that returns a list of valid tax classes. `WC_Product_Variation` then override this method, returning another list including the tax class 'parent'.
Fixes#17024