This commit removes support for CentOS 7 from the docs. The change applies to version 5.4,so it must be backported to branch-5.4. Refs https://github.com/scylladb/scylla-enterprise/issues/3502 In addition, this commit removes the information about Amazon Linux and Oracle Linux, unnecessarily added without request, and there's no clarity over which versions should be documented. Closes scylladb/scylladb#16279