Incorrect Postgres version number in README & on docker hub

[jschlicht]

From the README:

This image provides a single running Citus instance (atop PostgreSQL 12.2)

The image is currently running on top of PostgreSQL 16.1.

Could you update the readme?

I was about to make my own image when I noticed it was just out of date documentation.

The description on docker hub has the same text and should also be updated (if it isn't automatically set by a github action or integration of some kind).