CentOS 8 became Stream as of January 2022. Moreover, it looks like Stream 8 went EOL May 31. Going forward, you’d need to perform an in-place migration to Rocky Linux or AlmaLinux, migrating to Stream might work but - again, it’s EOL so no further security updates are delivered.
Change the repo URL from mirror.centos.org to vault.centos.org for all matching repos (ref: StackExchange):
sed -i s/mirror.centos.org/vault.centos.org/g /etc/yum.repos.d/*.repo
sed -i s/^#.*baseurl=http/baseurl=http/g /etc/yum.repos.d/*.repo
sed -i s/^mirrorlist=http/#mirrorlist=http/g /etc/yum.repos.d/*.repo
At this point, dnf should work once again allowing upcp migrations to complete. You can switch OS distros using cpcmd scope:set system.distro rocky (or “alma” for AlmaLinux) once everything is in proper working order. Rocky Linux and AlmaLinux will continue to be supported until May 2029.