Display correct plan name on Subscription Activation Form

What does this MR do?

This MR moves towards the implementation of this Issue gitlab-org/gitlab#273028 by addressing an inconsistency in the template where the plan name was hard-coded. The solution develops in 2 steps (MRs):

Description	MR
Add a helper to provide the view data	!57130 (merged)
Display correct plan name on Subscription Activation Form	👈 you are here

Screenshots (strongly suggested)

Before (Ultimate)	After (Ultimate)

How to test

If you want to check the feature,

• In your Rails console, activate Cloud License by using

ApplicationSetting.current.update(cloud_license_enabled: true)

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because the feature is not active.
• [-] 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