Swap COS8 image for UBI8 image

While we're here, add COS9 for EL9-based testing

Author: ferricoxide

.github/workflows/test-salt-linux.yml

@@ -39,7 +39,8 @@ jobs:
       image: >-
         ${{ fromJson('{
           7: "quay.io/centos/centos:centos7",
-          8: "quay.io/centos/centos:stream8"
+          8: "hub.docker.com/redhat:ubi8",
+          9: "quay.io/centos/centos:stream9"
         }')[inputs.salt-os-version] }}
       volumes:
         - /sys/fs/cgroup:/sys/fs/cgroup