Drop support for RHEL 7 / CentOS 7 (#11246)

* Simplify docker systemd unit

systemd handles missing unit by ignoring the dependency so we don't need
to template them.

* Remove RHEL 7/CentOS 7 support

- remove ref in kubespray roles
- move CI from centos 7 to 8
- remove docs related to centos7

* Remove container-storage-setup

Only used for RHEL 7 and CentOS 7

roles/kubernetes/preinstall/vars/main.yml

@@ -93,11 +93,8 @@ pkgs:
   python3-libselinux:
     os:
       distributions:
-        RedHat: &major_redhat_like
-          major_versions:
-          - "8"
-          - "9"
-        CentOS: *major_redhat_like
+        RedHat: {}
+        CentOS: {}
   rsync: {}
   socat: {}
   software-properties-common: *debian_family_base