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.
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