Feature/add flannel wireguard encryption backend as option (#9583)

* feat(): Add wireguard backend to flannel cni

As described in the flannel docs:
https://github.com/flannel-io/flannel/blob/master/Documentation/backends.md#wireguard

This does not support optional configuration methods like:
- setting a psk (will be autogenerated by default)
- chang listening ports
- change mode (defaults to 'separate')
- change PersistentKeepaliveInterval (defaults to 0)

* Add supported backends to flannel docs

* Fix markdown in docs

roles/network_plugin/flannel/tasks/main.yml

@@ -1,4 +1,13 @@
 ---
+
+- name: Flannel | Stop if kernel version is too low for Flannel Wireguard encryption
+  assert:
+    that: ansible_kernel.split('-')[0] is version('5.6.0', '>=')
+  when:
+    - kube_network_plugin == 'flannel'
+    - flannel_backend_type == 'wireguard'
+    - not ignore_assert_errors
+
 - name: Flannel | Create Flannel manifests
   template:
     src: "{{ item.file }}.j2"