Chart: Helm Refurbish

### Body As part of our ongoing Helm chart refurbish work in Apache Airflow, we will follow a structured approach to ensure stability for existing users while preparing for a clean, well-defined next major release. https://cwiki.apache.org/confluence/display/AIRFLOW/Helm+Refurbish https://cwiki.apache.org/confluence/display/AIRFLOW/Direction+and+Plan+for+Chart+2.0?src=contextnavpagetreemode Single tracker for the Helm chart 2.0 refurbishment. Each section below is a category of work; the items inside are the concrete action items. Tick as they land. Larger items may spawn dedicated PRs and issues — link them from the relevant action item rather than removing the checkbox. Contributor guide: Developing the Helm Chart (coming in #66659). Canonical chart-vs-Kustomize criteria: `chart/kustomize-overlays/CONTRIBUTING.rst`. The chart never removes a component without a working overlay already in place. --- #### Deprecate and remove legacy keys Deprecation warning ships on 1.2x for at least one minor before removal on main. - [ ] Deprecate `apiSecretKeySecretName` on 1.2x; remove in 2.0 - [ ] Deprecate `apiServer.apiServerConfig` on 1.2x; remove in 2.0 - [x] Remove misplaced ingress keys: `registry.secretNames`, `securityContext`, `ingress.apiServer.host`, `ingress.apiServer.tls.enabled`, `ingress.apiServer.tls.secretName`, `ingress.enabled` - [x] Stage all webserver-related deprecations on main - [ ] Audit remaining Airflow-below-3.0 compatibility keys; deprecate then remove #### Relocate parameters to their owning sections No functional change — parameters work today but live under the wrong parent. Canonical name changes. - [ ] Move Redis parameters under the celery section - [ ] Move Jobs parameters under the kubernetes section - [ ] Consolidate `data.celery.*` under a celery-specific heading - [ ] Align `podDisruptionBudget` shape with `pod_template_file` (Kubernetes executor pattern) - [ ] Audit remaining parameters under `kubernetes.…