Use plan name instead of plan code in new subscription app

What does this MR do?

It updates the new subscription to use plan name instead of plan code, it will have a better representation of the selected plan.

Related to gitlab-org/customers-gitlab-com#2600

Screenshots

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• 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.