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.
It would be better if the documentation could be generated on multiple operating systems all in one go, but the doc system currently does not allow vars to be changed once they are set.
The solution is to run the docs for each required OS and stitch the documentation together. It's not pretty but it works and the automation in release.pl should at least make it easy to use.
