Subject: [bug#48484] Removed unused PostgreSQL packages

From: Leo Famulari <[email-masked]>
Date: Mon, 17 May 2021 15:22:47 -0400

Currently we have packages for PostgreSQL 9.6, 10, 11, and 13.

Versions 9.6 and 11 are not used in Guix. So I propose that we remove
them.