When looking at the CI jobs on cirrus-ci.com, it seems like the mojave-based
images have been decomissioned a while ago already, since apparently all our
jobs get automatically upgraded to catalina. So let's update our YML script
accordingly to avoid confusion.

Reviewed-by: Ed Maste <emaste@freebsd.org>
Message-Id: <20200728074405.13118-5-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>