Since CentOS 8 will be EOL at the end of the year it makes sense to do this now. The centos:8 image is still used in documentation.xml because changes there require manual testing, which will need to be done at a later date. The changes are not user-facing, however, and can be done at any time.
Also update CentOS references to RHEL since that is what we are emulating for testing purposes.
The user guide was split primarily to provide documentation for the stop-auto option in PostgreSQL <= 9.5. Now that 9.5 is EOL there does not seem to be a good reason to generate an extra user guide. The stop-auto function is still documented in the reference.
Leave the stop-auto documentation in the user guide in case we want to manually generate documentation for older versions.
Also rename centos to rhel for most identifiers since that is the core platform we are building for, similar to how we label 'debian' builds even though we generally use Ubuntu. With CentOS set to become an upstream for RHEL later this year, we'll likely need to pick a new test distribution, perhaps Rocky Linux if that gets off the ground.
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.
