While not supported, you can use freeradius-postgresql by enabling the devel repository in Rocky temporarily, eg dnf install freeradius-postgresql --enablerepo=devel
The reason why you don’t see freeradius-postgresql normally is because it’s not provided by CentOS Stream 9 nor RHEL 9. But the package is built when freeradius is built either way. We do keep a devel repository around to ensure these packages can potentially be installed or used in some form in the event Red Hat does not provide them normally.