Prevent green check from showing on Billing Address

What does this MR do and why?

Follow-up from #432421 (closed)

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

purchase_flow_with_correct_check_marks

Subscription details	Contact information	Payment method	Completed

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:key_contacts_management)

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/new?plan_id=2c92c0f9699eca030169a2f1f117045f and test the same logic (add-on form)
7. Disable the feature flag

   Feature.disable(:key_contacts_management)

8. Verify the form still works as expected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.