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/bootstrap-os/molecule/default/molecule.yml

@@ -19,12 +19,6 @@ platforms:
     memory: 1024
     provider_options:
       driver: kvm
-  - name: centos7
-    box: centos/7
-    cpus: 1
-    memory: 512
-    provider_options:
-      driver: kvm
   - name: almalinux8
     box: almalinux/8
     cpus: 1