Docs: Move `HelmRepository` type `oci` to maintenance mode

[matheuscscp]

I'd like to update the Note we have in the HelmRepository type oci API doc section from this:

Note: For improved support for OCI Helm charts, please use the OCIRepository API.

To this:

Note: The oci type is in maintenance mode. For improved support for OCI Helm charts, please use the OCIRepository API instead. For more information, see #1955.

Reason

We are constantly advising users to move from HelmRepository type oci to OCIRepository. There are countless Slack threads where we tell the same story:

• .spec.chartRef in the HelmRelease API is an optimization over .spec.chart (allows reuse of chart objects)
• Users get surprised about the hidden HelmChart created by HelmRelease with .spec.chart and have a hard time understanding this is actually an optimization
• .spec.chartRef brings the HelmRelease deployment model closer to Kustomization (2 objects: 1 source + 1 applier)
• Less magic => less confusion
• Our main OCI efforts are focused on OCIRepository (e.g. Feature Request: Support OCIRepository for remote dependencies on umbrella charts defined with HelmChart objects that are built from file trees #1954)
• Users get confused by HelmRepository type oci being stateless: Helm OCI Repositories should have a ready status #1930
• I have blocked an issue in the Headlamp plugin to improve the support for HelmRepository type oci: Flux: 🐛 OCI Helm repos are shown as failed on overview headlamp-k8s/plugins#313

It has been raised probably more than once in the Slack threads that the docs should have more emphasis on this move we are asking users to make.