Use OCIRepository instead of HelmRepository for sylva-units

In order to enable the signature verification of sylva-unit helmrelease OCI, we have to use oci repositories as source for sylva-units.

For that purpose, we'll have to stop using valueFiles, and switch to valuesFrom as discussed in sylva-projects/sylva-core!3180 (comment 2332685064)

This should be fairly simple since we've implemented the capability to read from oci repositories in !294 (merged)