Confluence: Refactor Postgres subchart

The embedded Postgres support added in issue #40 (closed) is working, but relies on custom templates (db-deployment.yaml, db-service.yaml, etc.) maintained by the Big Bang team.

A better solution, and what other packages (GitLab, SonarQube, etc.) have implemented, is a subchart structure. The Postgres deployment should utilize an upstream chart instead of custom templates to reduce the overhead of maintenance work for future developers.

The subchart should be added as an OCI dependency and should still utilize the hardened Iron Bank image.