fix(postgres-system): Fix broken podMonitor namespace selector

If no namespace selector is sepcified, it will only search in the
current namespace. This is not intended for this podMonitor, therefore a
config to explicitly search all namespaces is required.

References:
https://github.com/prometheus-operator/prometheus-operator/blob/615162d6d35314f94cb9c69f51bc47d62b81f154/Documentation/api.md#namespaceselector