Customer code to mark_as_confirmed is not used at the moment

Problem

In the Customer model, there is a before_create callback that sets the Customer as confirmed if the zuora_account_id is set. This was introduced about 4 years ago in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/322+.

I found when removing the account_data_structure feature flag, that this code may not be working as originally intended anymore. This code requires the zuora_accound_id is set when the customer is being created to automatically set it as confirmed. However, the zuora_account_id method is overwritten in Customer so when this callback is executed, it may be using the overwritten method that pulls the zuora_account_id from the related billing_account, rather than the zuora_account_id column the Customer object. This column will eventually be removed anyway so either way this will need to be revisited.

Proposal

Instead of using a callback, we could explore updating the FindOrCreateCustomerService with logic that sets the object as confirmed (using mark_as_confirmed) if the account_id exists on create.