Update Cilium related docs (#18563)

* Extend Cilium CNI plugin description

Mention that it works on top of other CNI plugins due to the CNI
chaining [1].

[1]: http://docs.cilium.io/en/v1.6/gettingstarted/cni-chaining/

Signed-off-by: Martynas Pumputis <m@lambda.lt>

* Update Cilium installation steps for kubeadm

- Use the latest v1.6 Cilium.
- By default, Cilium no longer needs/deploys ETCD store.

Signed-off-by: Martynas Pumputis <m@lambda.lt>

* Update Cilium NetworkPolicy guide

- Update minikube min version requirement.
- Use Cilium v1.6.
- Remove the etcd store bits, as Cilium v1.6 no longer depend on
  it by default.

Signed-off-by: Martynas Pumputis <m@lambda.lt>

* Update Cilium installation steps for kubeadm

- How to run without kube-proxy.

Signed-off-by: Martynas Pumputis <m@lambda.lt>

Co-authored-by: Martynas Pumputis <m@lambda.lt>

content/en/docs/concepts/cluster-administration/networking.md

@@ -125,7 +125,7 @@ BCF was recognized by Gartner as a visionary in the latest [Magic Quadrant](http
 providing and transparently securing network connectivity between application
 containers. Cilium is L7/HTTP aware and can enforce network policies on L3-L7
 using an identity based security model that is decoupled from network
-addressing.
+addressing, and it can be used in combination with other CNI plugins.
 
 ### CNI-Genie from Huawei