System unavailable: RHEL6/x64

[sxa]

RHEL6 systems have been marked offline since JDK20+ won't run on them now that adoptium/ci-jenkins-pipelines#627 is in. Once a remediation for the test jobs is in, they can be re-enabled. This issue is a placeholder to remember to do that.

Affected systems:

• https://ci.adoptium.net/computer/test%2Dibmcloud%2Drhel6%2Dx64%2D1/
  Note that trying to use !rhel6 in the ADDITIONAL_TEST_LABELS field does not work::

10:39:56  LABEL: ci.role.test&&hw.arch.x86&&sw.os.linux&&!(rhel6)
[Pipeline] stage
[Pipeline] { (Queue)
[Pipeline] nodesByLabel
10:39:56  Could not find any nodes with 'ci.role.test&&hw.arch.x86&&sw.os.linux&&!(rhel6)' label

despite the jenkins view of https://ci.adoptium.net/label/ci.role.test&&hw.arch.x86&&sw.os.linux&&!rhel6/ showing the expected thing.
Worst case we may need a new capability label for "not rhel6" or just stop testing on RHEL6