Include default plan as being eligible for a trial

What does this MR do?

Adds the default plan as one which is eligible for a trial. The default plan is essentially what you get when you don’t have a plan at all, especially on CE. For EE, we have the free plan, but there’s no guarantee that every single record has some plan set. They might have nil, default, free, or any of the “hosted” options.

I don’t think there’s any need for a changelog entry or updated documentation as this is mostly a developer-facing change.

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