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.

ci/cyberchef/test-values.yaml

@@ -1,5 +1,6 @@
-# cyberchef: stateless, no secrets. Only the ingress host is required
-# (clusterIssuer already has a default). The Ingress just has to apply -
-# there is no ingress-nginx/cert-manager in the CI cluster.
+# cyberchef: stateless, no secrets. Only the required ingress values are set.
+# The Ingress just has to apply - there is no ingress-nginx/cert-manager in
+# the CI cluster.
 ingress:
   domain: cyberchef.ci.local
+  clusterIssuer: ci

ci/urlaubsverwaltung/test-values.yaml

@@ -4,3 +4,6 @@ secrets:
 
 ingress:
   domain: uv.ci.local
+  # clusterIssuer is strictly required (no default); the Ingress just has to
+  # apply - there is no cert-manager in the CI cluster.
+  clusterIssuer: ci