Try to make ancestor_clusters_for_clusterable return a relation

What does this MR do?

1. ancestor_clusters_for_clusterable should return a relation instead of an array
2. Once 1 is done, we can then combine project, group and instance into a UNION query

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/63475

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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