support Rocky Linux in mariadb_repo_setup script

From https://mariadb.com/kb/en/no-rocky-linux-support-yet/ :

the only way i was able to get RL to run was to look for any reference in the ./mariadb_repo_setup script and any place i saw rhel|centos i changed it to rhel|centos|rocky and also changed /etc/redhat-release from Rocky LInux to Centos Linux.

when will Rocky-Linux be fully supported? I don't feel comfortable hacking an install script to get it to work.

also i would suggest changing yum to dnf