Experiment cleanup: Update /billing page to mirror /pricing page design for Free users

Summary

The experiment is currently rolled out to 100% of users and has been deemed a success. The changes need to become an official part of the product.

• Experiment key / feature flag name: promote_premium_billing_page
• Epic or issue link: https://gitlab.com/groups/gitlab-org/-/epics/7173
• Rollout issue https://gitlab.com/gitlab-org/gitlab/-/issues/346275

Improvements to include as part of the cleanup

• Currently the tier feature values on the page are hard-coded as part of the cleanup we should work with Fulfillment to identify and utilize a shared resource that updates the tier feature values on the page, see related comment from Doug: https://gitlab.com/gitlab-org/gitlab/-/issues/346273#note_911966924

Steps

• Determine whether the feature should apply to SaaS and/or self-managed
  • SaaS and self-managed
• Determine whether the feature should apply to EE - and which tiers - and/or Core
  • This change should be shipped to EE free instances and groups. The reason we're excluding paying customers is the paid billing page contains specific information related to their subscription and seat usage which this page design does not contain.
• Determine if tracking should be kept as is, removed, or modified.
  • Tracking should be kept
• Ensure any relevant documentation has been updated.
• Consider changes to any feature_category: introduced by the experiment if ownership is changing (PM for Growth and PM for the new category as DRIs)
• Check to see if the experiment introduced new design assets. Add them to the appropriate repos and document them if needed.
• Optional: Migrate experiment to a default enabled feature flag for one milestone and add a changelog. Converting to a feature flag can be skipped at the ICs discretion if risk is deemed low with consideration to both SaaS and (if applicable) self managed
• In the next milestone, remove the feature flag if applicable
• After the flag removal is deployed, clean up the feature/experiment feature flags by running chatops command in #production channel
• Ensure the corresponding Experiment Rollout issue is updated