Promote `HelmRelease` API to v2 (GA)

[stefanprodan]

This PR promotes the HelmRelease API from v2beta2 to v2 and deprecates the v2beta2 API (to be removed after a period of 6 months).

⚠️ Removed Fields

In the helm.toolkit.fluxcd.io/v2 API the following fields (deprecated since v2beta1) are removed:

• .spec.chart.spec.valuesFile replaced by .spec.chart.spec.valuesFiles
• .spec.postRenderers.kustomize.patchesJson6902 replaced by .spec.postRenderers.kustomize.patches
• .spec.postRenderers.kustomize.patchesStrategicMerge replaced by .spec.postRenderers.kustomize.patches
• .status.lastAppliedRevision replaced by .status.history.chartVersion

🏷️ New Fields

• .spec.chartRef allows referencing chart artifacts from OCIRepository and HelmChart objects.
• .spec.chart.spec.ignoreMissingValuesFiles allows ignoring missing values files instead of failing to reconcile.

📦 Helm OCI Artifacts

Starting with this version, the recommend way of using Helm OCI, is by referencing an OCIRepository object.

Helm OCI example:

apiVersion: source.toolkit.fluxcd.io/v1beta2
kind: OCIRepository
metadata:
  name: podinfo
  namespace: default
spec:
  interval: 10m
  layerSelector:
    mediaType: "application/vnd.cncf.helm.chart.content.v1.tar+gzip"
    operation: copy
  url: oci://ghcr.io/stefanprodan/charts/podinfo
  ref:
    semver: "*"
---
apiVersion: helm.toolkit.fluxcd.io/v2
kind: HelmRelease
metadata:
  name: podinfo
  namespace: default
spec:
  interval: 10m
  chartRef:
    kind: OCIRepository
    name: podinfo

Part of: https://github.com/fluxcd/flux2/issues/4712

[stevo-f3]

custom metrics example on the website is out of date https://fluxcd.io/flux/monitoring/custom-metrics/#adding-custom-metrics https://github.com/fluxcd/website/blob/main/content/en/flux/monitoring/custom-metrics.md?plain=1#L106

Needs a fix like https://github.com/fluxcd/flux2-monitoring-example/pull/36 i.e. https://github.com/fluxcd/flux2-monitoring-example/blob/main/monitoring/controllers/kube-prometheus-stack/kube-state-metrics-config.yaml#L72