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

test-infra/image-builder/roles/kubevirt-images/defaults/main.yml

@@ -48,13 +48,6 @@ images:
     converted: true
     tag: "latest"
 
-  centos-7:
-    filename: CentOS-7-x86_64-GenericCloud-2009.qcow2
-    url: http://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud-2211.qcow2
-    checksum: sha256:284aab2b23d91318f169ff464bce4d53404a15a0618ceb34562838c59af4adea
-    converted: true
-    tag: "latest"
-
   centos-8:
     filename: CentOS-8-GenericCloud-8.3.2011-20201204.2.x86_64.qcow2
     url: http://cloud.centos.org/centos/8/x86_64/images/CentOS-8-GenericCloud-8.3.2011-20201204.2.x86_64.qcow2