Use ocirepositories.spec.certificatesStore instead of mounting extra-ca in source controller

We are currently using that component to mount oci registry certificates: https://gitlab.com/sylva-projects/sylva-core/-/blob/1.4.1/kustomize-units/flux-system/components/extra-ca/kustomization.yaml?ref_type=tags

The issue has been fixed in flux: https://github.com/fluxcd/flux2/issues/3417

We should leverage that new feature for that purpose.