Withdrawal of CentOS 8 test container support

[nitzmahone]

Now that CentOS 8 has been fully discontinued, most package mirrors have begun culling CentOS 8 content, which breaks all ansible-test CentOS 8 containers configured on discontinued production package mirrors. Rather than rebuilding all versions of the affected containers to use the vault.centos.org package mirrors or moving to a clone distro, we're just discontinuing test container support for CentOS 8. This decision is based largely on the fact that RHEL 8 test VM support via ansible-core-ci is available on all branches to provide nearly identical test coverage.

We recommend that projects immediately remove any centos8/ ansible-test container entries in their CI matrices, as they will likely fail on tasks that require access to a package mirror. Projects running their CI under Ansible's Azure Pipelines subscription can replace those CentOS 8 matrix entries with rhel8.2/-rhel8.5/ (depending on which version of ansible-test is in use) to get nearly identical coverage under a RHEL 8 VM, provided and paid for by Ansible.

If you want to discuss this, please write your comments here: #4