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/ingress.yaml

@@ -14,7 +14,7 @@ metadata:
   name: {{ include "gitea.ingress.name" . }}
   namespace: {{ .Release.Namespace }}
 spec:
-  ingressClassName: {{ tpl .Values.ingress.className . }}
+  ingressClassName: {{ tpl .Values.ingress.ingressClassName . }}
   rules:
     {{- range .Values.ingress.hosts }}
     - host: {{ tpl .host $ | quote }}