tests: Update our CI to use CentOS Stream 9 instead of 8

tests: Update our CI to use CentOS Stream 9 instead of 8

RHEL 9 (and thus also the derivatives) have been available since two
years now, so according to QEMU's support policy, we can drop the active
support for the previous major version 8 now.

Another reason for doing this is that Centos Stream 8 will go EOL soon:

https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-stream-8-and-centos-linux-7/

  "After May 31, 2024, CentOS Stream 8 will be archived
   and no further updates will be provided."

Thus upgrade our CentOS Stream container to major version 9 now.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20240418101056.302103-5-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>