Update RHEL/CentOS 7 to cover the versions that were previously covered by RHEL/CentOS 6.
Since RHEL/CentOS 7/8 work the same update the documentation logic and labels to reflect this compatibility.
CentOS6 EOL'd and the mirrors were swiftly deleted, leading to failures in tests and documentation.
Remove CentOS 6 for now to get builds going again with the intention to replace it in the near future with CentOS 8.