Update the default downloads site URL to https://www.chef.io/downloads

[tas50]

We're migrated from downloads.chef.io to https://www.chef.io/downloads. We need to make sure to update the default URL in Supermarket.

[btm]

https://github.com/chef/supermarket/blob/main/src/supermarket/app/helpers/custom_url_helper.rb#L40-L43