feat(ingress)!: rename className to ingressClassName

The value only ever populated the `spec.ingressClassName` field of the Ingress resource, so the shortened name was a
needless deviation from the Kubernetes API. `ingress.className` carried a `DEPRECATED` marker since #679, but that
marker was a leftover: the rename to the canonical name was proposed there, rejected during review and the follow-up
commits only removed the half-merged template fallback. The marker stayed behind for a year while the value remained
the one and only way to set the Ingress class, which made the documentation actively misleading.

The deprecation guard uses `hasKey` instead of a truthiness check on purpose. The previous chart default was an empty
string, so vendored values files tend to carry `ingress.className: ""` verbatim. A truthiness check would silently
ignore those and leave users wondering why their Ingress class is no longer applied.

Co-authored-by: Copilot <copilot@github.com>

templates/deprecation.yaml

@@ -70,6 +70,11 @@
     {{- fail "`extraVolumes` does no longer exist. Please refer to the changelog and configure `deployment.volumes` instead." -}}
   {{- end -}}
 
+  {{/* INGRESS CLASS NAME */}}
+  {{- if hasKey .Values.ingress "className" -}}
+    {{- fail "`ingress.className` does no longer exist. Please refer to the changelog and configure `ingress.ingressClassName` instead." -}}
+  {{- end -}}
+
   {{/* NODE SELECTOR */}}
   {{- if .Values.nodeSelector -}}
     {{- fail "`nodeSelector` does no longer exist. Please refer to the changelog and configure `deployment.nodeSelector` instead." -}}