Cleaning up certificate-based clusters

I maintain a self-hosted CE instance, running with gitlab/gitlab-ce:15.0.0-ce.0.

Following the deprecation of certificate-based cluster registration, I’m trying to clean up our cluster integrations. While I have manually removed those (as well as associated deployment environments) I was aware of, I still see a few clusters when running

Clusters::Cluster.all

in the gitlab-rails console. Based on the information that shows, I get the impression that they belonged to projects that have been deleted. I want to make sure so how do I find the projects (or groups) that clusters belong too?

If they do belonged to projects/groups since deleted, how come they weren’t cleaned up and how would I get rid of them?