Switch to using a hard-coded release version for RHEL repo configs. (#21649)
'$releasever_major' is a newer thing than we initially realized, and thus broke compatibility with RHEL 9.0 through 9.6 (and equivalent platforms). We had only ever been using the `$releasever` variable to support clean handling of in-place distribution release upgrades (for example, upgrading from RHEL 9 to RHEL 10 without a clean reinstall of the whole system), but in reality that’s a relatively rare use-case on RHEL systems, so there’s not much advantage to continuing to support it there as doing so would make it significantly more complicated to make the repository configuration work with both older and newer versions of RHEL. Instead, just use the hardcoded major version of the release in the repository config for these platforms. Fedora, openSUSE, and Oracle Linux will continue to use `$releasever` as they had been, as it continues to work correctly there.