The Helm chart on GHCR displays a `docker pull` command instead of
the correct `helm pull oci://` command. This is because the OCI artifact
is missing the `org.opencontainers.image.source` annotation that GHCR
uses to identify and properly display Helm charts.
Changes:
- Add OCI annotations to Chart.yaml (source + url) which Helm 3.10+
propagates to the OCI manifest on push
- Install explicit Helm v3.20.0 via azure/setup-helm@v4 for reproducible
builds and proper OCI annotation support
- Remove deprecated HELM_EXPERIMENTAL_OCI env var (OCI is GA since Helm 3.8)