prometheus-adapter image ref in CMO manifests is outdated

We set image links on CMO's jsonnet code, as these can sometimes be used to populate labels and is generally considered good documentation pratice.

In a cluster these links are replaced by CVO.

prometheus-adapter is now a k8s project and has moved locations accordingly from directxman12/k8s-prometheus-adapter to kubernetes-sigs/prometheus-adapter. This should be reflected in our image links, set at https://github.com/openshift/cluster-monitoring-operator/blob/35a063722c7e3b68d57aed18dc81f0dbdfbfc004/jsonnet/main.jsonnet#L66.