Remove EOL CentOs-7 and CentOs-8 from metadata.json

[cathal41]

Summary

Removing CentOs-7 and CentOs-8 from metadata.json as the CI and Nightly tests are failing as CentOs-8 is no longer supported and CentOs-7 will reach EOL on the 30th of June 2024

Additional Context

CentOs-8 is no longer supported by PE and no longer available to be provisioned as a VM Tests will continue to run on redhat-8, centOs-8 is a derivative of redhat-8

Checklist

• [ ] 🟢 Spec tests.
• [ ] 🟢 Acceptance tests.

Changes include test coverage?

• [ ] Yes
• [ ] Not needed

Have you updated the documentation?

• [ ] Yes, I've updated the appropriate docs
• [ ] Not needed

[bastelfreak]

@cathal41 can you maybe add a meaningful PR title before we merge it?