Use `find_each` rather than `all.each`

Perform the CreateDefaultStock migration in batches to lower memory footprint.
[sample] Use new preferences syntax in shipping_methods
Using `find_each` instead of `all.each` to improve memory

This syntax will run our migration over the `Spree::StockTransfer` model in batches, which will improve performance on stores with high numbers of stock transfers.

Fixes #4473