Allow namespaces in a trial to upgrade to any plan

What does this MR do?

Currently namespaces in a trial of do not have the upgrade button. In addition namespaces in a silver or bronze trial have wrong URLs.

Trialed namespaces can not upgrade because the purchase URL returned by the customers portal does not have a purchase link.

This change assures a trialed namespaces is indeed a free namespace as when offering an upgrade.

Reference: The call to the anonymous API: https://customers.gitlab.com/gitlab_plans?plan=free

Issue #122398 (closed)

Screenshots

Before	After
Buggy URL https://gitlab.com/groups/tombak/-/&gl_namespace_id=67	Correct upgrade URL: http://customers.gitlab.com/subscriptions/new?plan_id=2c92c0f95aac74f3015aaea8d97a4b48&gl_namespace_id=64

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

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