Update charts default postgresql to PG 16

Summary

In GitLab 18.0, support for Postgres 14 is being dropped, and support for PG 16 is intended to be added.

In the the GitLab 17.11 milestone we should

• complete an investigation and POC of updating our bitnami postgres chart to the newest version
• Investigate whether our chart DB upgrade steps are viable for jumping from pg14 to 16 (doing a two version jump allows us to only force our users to do this every two years in charts for those using the provided database)

In the Gitab 18.0 milestone we need to:

• Update the PG min-version checks to PG 16
• Set the default postgres image version to 16
• Update the bitnami postgres chart if feasible
• Ensure all the db upgrade documentation is up to date and accurate for the chart