Rename NamespaceStorageLimitAlertHelper method

What does this MR do?

Part of #280862 (closed)

This MR renames a helper method in EE::NamespaceStorageLimitAlertHelper from can_purchase_storage_for_namespace? to purchase_storage_link_enabled? for clarity. The previous name gave the impression this involved user abilities which it does not. It also removes a method, can_purchase_storage?, that is no longer used.

I did not add a changelog entry for this small change as it was developer-facing (refactoring).

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
• [-] 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 #280862 (closed)