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/tasks/main.yml

@@ -130,16 +130,6 @@
   tags:
     - bootstrap-os
 
-- name: Grow partition on azure CentOS
-  import_tasks: 0120-growpart-azure-centos-7.yml
-  when:
-    - not dns_late
-    - azure_check.stat.exists
-    - ansible_os_family == "RedHat"
-    - growpart_azure_enabled
-  tags:
-    - bootstrap-os
-
 - name: Run calico checks
   include_role:
     name: network_plugin/calico