Upgrade zulip postgres to v14
We discussed this in the context of #1136 (closed) and agreed this is not relevant for %0.8.0. I created https://github.com/zulip/zulip/issues/21211 to ask for a postgres 14 zulip-postgres image nonetheless so lets see when a new official image will be provided, and how the upgrade path looks like.
Right now it looks like zulip comes with an integrated easy postgres upgrade procedure, although the zulip-postgres Dockerfile says otherwise:
Currently the PostgreSQL images do not support automatic upgrading of the on-disk data in volumes. So the base image can not currently be upgraded without users needing a manual pgdump and restore.