Omit sub-groups from trial eligibility

What does this MR do?

Adds a condition to the Namespace.eligible_for_trial scope so that only top-level namespaces are returned. Previously, we returned all namespaces, including sub-groups, belonging to the current user, but we only allow trials on top-level namespaces.

Screenshots

With the following configuration of groups:

This MR takes the trial group selection menu from Before to After:

Before	After

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

Addresses #228781