How do I have to configure certmanaager when using a gitlab managed cluster?

This error message comes from the fact that you are using cert-manager v0.10.1 or below with Kubernetes 1.20 or above. The issue disappears with cert-manager v0.11.0.

I would encourage you to use a recent version of cert-manager. I noticed that helmfile.yaml has both an old and recent version of the cert-manager chart; cert-manager-1-4 is the one you should use.

helmfiles:
#  - path: applications/cert-manager/helmfile.yaml     # ❌ cert-manager v0.10.1
#  - path: applications/cert-manager-1-4/helmfile.yaml # ✅ cert-manager v1.4.0

Source: https://github.com/jetstack/cert-manager/issues/3615