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/ingress/ingress.yaml

@@ -90,6 +90,18 @@ tests:
               - git.example.com
             secretName: tls-secret
 
+  - it: Render ingress with a custom ingressClassName
+    set:
+      ingress.enabled: true
+      ingress.ingressClassName: traefik
+      service.http.enabled: true
+    asserts:
+      - hasDocuments:
+          count: 1
+      - equal:
+          path: spec.ingressClassName
+          value: traefik
+
   - it: Render ingress with custom HTTP service spec
     set:
       ingress.enabled: true