Use Cloud Native Postgres instead of bitnami

[zkutasi]

I tried to pull up Tianji just to check it, but I have seen by default it tries to pull a bitnami postgresql image

  Normal   BackOff                 3s (x9 over 2m35s)   kubelet                  Back-off pulling image "docker.io/bitnami/postgresql:16.3.0-debian-12-r10"

It would be better if you could assume CloudNative PG is present in the cluster, and use a CNPG instance... CNPG is nowadays the de-facto way to run Postgres.