Remove el8 support (glibc too old for recent oc CLI)

[elfosardo]

Recent OpenShift releases require glibc 2.34+ which is only available in el9. The oc CLI from OCP 4.14+ won't run on el8.

• Remove el8 case from install requirements
• Update validation to require version >= 9
• Remove oc.rhel8 extraction fallback
• Simplify CI Dockerfile to RHEL9 only
• Update README prerequisites

[openshift-ci]

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[elfosardo]

/cc @hroyrh @tdomnesc @honza

[openshift-ci]

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[elfosardo]

/retest

[elfosardo]

/retest

[elfosardo]

/retest

[elfosardo]

/retest

[elfosardo]

/retest

[elfosardo]

/retest

[openshift-ci]

@elfosardo: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-agent-compact-ipv4-iso-no-registry	6be8554	link	false	/test e2e-agent-compact-ipv4-iso-no-registry

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