Refactor EE find_group_cluster_platform_kubernetes

What does this MR do?

Refactor EE find_group_cluster_platform_kubernetes to return the first cluster so that we can sort ascending.

This should enable us to not use hierarchy_order in the future as CTE defaults to ascending order.

This is possible thanks to a new option relevant_only in the on_environment scope

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