System unavailable: RHEL6/x64

RHEL6 systems have been marked offline since JDK20+ won't run on them now that https://github.com/adoptium/ci-jenkins-pipelines/pull/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:](https://ci.adoptium.net/job/Test_openjdk20_hs_sanity.functional_x86-64_linux/75/):
```
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

Review ci-adoptium/ci-jenkins-pipelines#627, the affected RHEL6 Jenkins node, and the Test_openjdk20_hs_sanity.functional_x86-64_linux/75 job output. Investigate why the !rhel6 label is not selecting nodes and determine a remediation for the test jobs. Done means the RHEL6 systems can be re-enabled, or the project has decided to stop testing on them.

Written by the indexing model from the issue text.