[Key Contacts Management] Direct the user to update contact information in CustomersDot when billing account is present (already has a billing account contacts set up)

When the billing account information from the customer is fetched, we should update the billing address step from STEPS in the step_order_app.vue that is used both by buy_addons_shared/components/app.vue and subscriptions/new/components/app.vue to direct the user to update their contact information in CustomersDot, if they need to change it.

See more information in the breakdown here: customers-gitlab-com#7991 (comment 1653715044) and the discussion here: https://gitlab.com/gitlab-org/fulfillment/meta/-/issues/1553#note_1658892477.