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>

unittests/helm/deployment/deprecations.yaml

@@ -63,6 +63,12 @@ tests:
     asserts:
       - failedTemplate:
           errorMessage: "`extraVolumes` does no longer exist. Please refer to the changelog and configure `deployment.volumes` instead."
+  - it: fails when the removed `ingress.className` value is set
+    set:
+      ingress.className: ""
+    asserts:
+      - failedTemplate:
+          errorMessage: "`ingress.className` does no longer exist. Please refer to the changelog and configure `ingress.ingressClassName` instead."
   - it: fails when the removed `nodeSelector` value is set
     set:
       nodeSelector: