Keep the versions of metallb in sync

The following discussion from !370 (merged) should be addressed:

> I think that it would be important, as a follow-up to this MR to ensure that we keep all our metallb versions in sync:
> * the version here
> * the version in `kustomize-units/cluster-manifests/rke2-capo/base/rke2-capo-cluster.yaml`
> * the version in `kustomize-units/metallb/kustomization.yaml` (`https://github.com/metallb/metallb/config/native?ref=v0.13.9`)

The three places in which we have metallb are:

• tools/oci/push-helm-charts-artifacts.sh (Helm chart)
• the version in kustomize-units/cluster-manifests/rke2-capo/base/rke2-capo-cluster.yaml (Helm chart)
• the version in kustomize-units/metallb/kustomization.yaml (https://github.com/metallb/metallb/config/native?ref=v0.13.9) (Kustomization)

cc @feleouet