Thanks very much Devrim, that got me on the right track.
> Please update the repo RPM to the *latest* version first (42.0-13 or
> above). rc packages were in the testing repo. v13 packages went to the
> stable repo, which I added in 42.0-13.
I had 42.0-14 installed but I had to manually uninstall and re-install to get the stable repo in the list in /etc/yum.repos.d/pgdg-redhat-all.repo
rpm -qa | grep pgdg
rpm -e pgdg-redhat-repo-42.0-14.noarch
dnf install -y https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm
Jack