Change how sylva-units uses OCI artifacts for Helm-based units

(follow-up to discussion)

We need to change how sylva-units uses OCI artifacts for Helm-based units.

We have been using the "HelmRepository of type OCI" approach, but with this approach signature verification is not implemented by FluxCD.

We need to use the "use an OCIRepository chartRef" (https://fluxcd.io/flux/guides/helmreleases/#using-a-chart-reference).

This approach is much better (it did not exist yet when we introduced OCI support in sylva-units).

Using OCIRepository for HelmReleases is necessary to address #2269.