SRE-3829 ci: Remove EL8 support for jenkins and GHA

[osalyk]

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

Required:

• SRE-3829 ci: remove EL8 from weekly testing #18556
• SRE-3829 ci: remove EL8 from weekly testing #18555
• SRE-3829 ci: remove EL8 from daily testing #18554
• SRE-3829 ci: remove EL8 from daily testing #18553

[github-actions]

Errors are Unable to load ticket data
https://daosio.atlassian.net/browse/SRE-3829

[daltonbohning]

Are we sure nobody is using EL8 internally and installing DAOS RPMs? If so, maybe we need to make the team aware of this change.

[JohnMalmberg]

Daily and weekly tests for master / release 2.8 may have VM testing stages for EL-8, and if so, they will need to be removed before this is landing.

Using EL-8 for hardware CI will cause issues that I have not yet deployed a bug fix for that. CI has not tripped on that bug, but some of the reservable LAB systems have.