Hi,
currently, Postgres 12 is the only supported version for Katello, that is correct.
Though, since you are using Katello, I assume you are on some EL/RHEL derivate, which means that Postgres 12 should be maintained by your distribution. If you are on RHEL, you can see here that Postgres 12 will be supported by RedHat until the end of the RHEL8 lifecycle. I am not sure about Alma, Rocky, etc, but since those distros claim is that they are 100% compatible to RHEL, I would assume they have the same policy.
So, even though Postgres 12 might be EOL soon upstream, RedHat or your distribution will continue to support it.