Feat(Purchase Flow): migrate step components to use GraphQL

What does this MR do?

This MR refactors step.vue, step_header.vue, step_summary.vue components to use a GraphQL local resolver instead of a Vuex store.

These components are used on the http://localhost:3000/-/subscriptions/new-page and will be used on http://localhost:3000/-/subscriptions/buy_minutes.

Screenshots (strongly suggested)

This still works as before

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because there is no user-facing change.
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #321650 (closed)