patroni liveness probe should use the /liveness endpoint

Issue is that the liveness probe currently only checks for Postgres being up, not Patroni

https://patroni.readthedocs.io/en/latest/rest_api.html#health-check-endpoints

cas-postgres `statefulset-patroni.yaml:168` should use `/liveness` as per the docs

Past discussion on YT, GGIRCS-2423:
> Forgot about this: https://srcco.de/posts/kubernetes-liveness-probes-are-dangerous.html
> Also those liveness and readiness endpoints are not supposed to be a thing until patroni 2, and we're on 1.6

> blocking this for now, I think that we need a readiness probe and alerts for pods that don't get ready, but we can discuss that next sprint

> Sounds good. It might be a bigger thing that we have to implement ourselves unfortunately :(