[18.6] Fix RHEL 10 not working with SELinux

What does this MR do and why?

This backports !8954 (merged) to 18-6-stable.

This commit refactors SELinux RHEL version checks to use a version comparison. Replace individual RHEL version checks (RHEL 7, 8, 9, 10) with a system_is_supported_rhel? method that performs numeric comparison. This simplifies the selinux_supported? check.

Relates to #9471 (closed)

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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