Update postgres helm chart to latest version

We didn't upgrade postgres in ~1,5y and are now facing 2 major version upgrade:

-  version: 8.6.4                                                                           
+  version: 10.9.1  

See https://github.com/bitnami/charts/tree/master/bitnami/postgresql#upgrading for a summary of breaking changes.

Imo we should do this ASAP before we get more users.

I already tried upgrading in !41 (merged) but hit this and reverted:

Last Helm logs:

Looks like there are no changes for Service "nc-rabbitmq-headless"
Looks like there are no changes for Service "nc-redis-headless"
Looks like there are no changes for Service "nc-redis-master"
error updating the resource "nc-postgresql":
   cannot patch "nc-postgresql" with kind StatefulSet: StatefulSet.apps "nc-postgresql" is invalid: spec: Forbidden: updates to statefulset spec for fields other than 'replicas', 'template', and 'updateStrategy' are forbidden
warning: Upgrade "nc" failed: cannot patch "nc-postgresql" with kind StatefulSet: StatefulSet.apps "nc-postgresql" is invalid: spec: Forbidden: updates to statefulset spec for fields other than 'replicas', 'template', and 'updateStrategy' are forbidden
  Normal  error  39s  helm-controller  reconciliation failed: Helm upgrade failed: cannot patch "nc-postgresql" with kind StatefulSet: StatefulSet.apps "nc-postgresql" is invalid: spec: Forbidden: updates to statefulset spec for fields other than 'replicas', 'template', and 'updateStrategy' are forbidden