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>

values.yaml

@@ -587,7 +587,7 @@ gatewayAPI:
 ## @param ingress.enabled Enable ingress.
 ## @param ingress.annotations Additional annotations.
 ## @param ingress.labels Additional labels.
-## @param ingress.className DEPRECATED: Ingress class name.
+## @param ingress.ingressClassName Name of the IngressClass which backs the Ingress.
 ## @param ingress.pathType Ingress Path Type.
 ## @param ingress.hosts[0].host Default Ingress host.
 ## @param ingress.hosts[0].paths[0].path Default Ingress path.
@@ -596,7 +596,7 @@ ingress:
   enabled: false
   annotations: {}
   labels: {}
-  className: "nginx"
+  ingressClassName: "nginx"
   pathType: Prefix
   hosts:
     - host: git.example.com