Investigate updating stable/postgresql to newer version

Summary

We currently employ a version of the stable/postgresql chart that does not support securityContext, which impacts some users (see #825 (closed)). We should investigate updating the upstream charts newer versions for compatibility with this chart, and update if possible.

From https://gitlab.com/charts/gitlab/issues/825#note_117645380

As of 419bb5b7

• This chart uses stable/postgresql 0.12.0(93ccc2350550ff5d95f13a5c25dc54babc44c048) which uses a single Deployment
• Upstream stable/postgresql @ master is 2.6.0 and has moved to bitnami based container(s).
• 0.13.x added direct support Prometheus metrics.
• 0.16.0 brought support for pg_hba.conf configuration.
• 2.0.x saw the take-over by Bitnami, which included support for securityContext