openshift-ci: switch to CentOS Stream
The build root container is switched from CentOS 8 to Stream 8 as
the former reached EOL.
Fixes
#
# This is the build root image for Kata Containers on OpenShift CI.
#
-FROM registry.centos.org/centos:8
+FROM quay.io/centos/centos:stream8
RUN yum -y update && \
yum -y install \
