Prevent premature completed state on Billing Address

Re-work of !139603 (merged)

Related to #434902 (closed)

Adds logic to only show a green check next to completed steps that have already been accessed. Previously, we were showing a green check next to Contact information before the user had accessed that step because no action was needed.

Implements a way to keep track of the furthest step so that we can determine if the user has already accessed the step or not.

Screenshots or screen recordings

New subscription	Add on
purchase_flow_with_correct_check_marks	add_on_subscription

Subscription details	Contact information	Payment method	Completed

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:key_contacts_management_v2, type: :gitlab_com_derisk)

2. Have CustomersDot set up and connected to GitLab
3. Visit http://localhost:3000/-/subscriptions/new?plan_id=2c92c0f876e0f4cc0176e176a08f1b70 (new subscription)
4. Go through the form,
5. Hit next as well as edit buttons to ensure the form works as expected
6. Visit http://localhost:3000/-/subscriptions/buy_minutes?selected_group=GROUP_ID_HERE and test the same logic (add-on form)
7. Disable the feature flag

   Feature.disable(:key_contacts_management_v2, type: :gitlab_com_derisk)

8. Verify the form still works as expected