Remove distros which have reached end of life, and remove some
commented-out entries for versions which have already been listed
elsewhere. Promptly trimming EOL versions makes our releases faster
since we need not upload packages for those versions.
Note that we remove CentOS 5 because we're no longer building packages
for it. Since RHEL is not freely available, we must rely on building
packages for CentOS, which means we must use CentOS's EOL dates, not
RHEL's.
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