Strict clusterIssuer also for cyberchef and urlaubsverwaltung

Per the maintainer decision (option a: ingress.clusterIssuer strictly
required in ALL charts) the two remaining defaults are removed as well:

- cyberchef: drop the letsencrypt default; bump raised from minor to
  major (6.0.0+up11.4.0) as this is a breaking values change.
- urlaubsverwaltung: drop the letsencrypt-issuer default; major bump
  to 6.0.0+up6.7.0.
- CI test-values for both charts now set clusterIssuer explicitly.

A repo-wide grep confirms no chart carries a clusterIssuer default
anymore.

urlaubsverwaltung/values.yaml

@@ -10,7 +10,7 @@ secrets:
 
 ingress:
   enabled: true
-  clusterIssuer: "letsencrypt-issuer"
+  clusterIssuer: null
   domain: null
   className: nginx
   # Extra annotations for the Ingress; merged with the cert-manager