cert-manager: upgrade to v1.20.4

One minor at a time from v1.15.5 to v1.20.4 on the way to Kubernetes 1.33, see docs/upgrade-k8s-1.33.md. The chart renders the current values unchanged.

apps/templates/cert-manager-helm.yaml

@@ -12,7 +12,7 @@ spec:
   project: default
   source:
     repoURL: https://charts.jetstack.io
-    targetRevision: v1.19.6
+    targetRevision: v1.20.4
     chart: cert-manager
     helm:
       parameters: