Remove log4j-kubernetes in version 2.x

Due to differences in the lifecycle of Log4j Core and Kubernetes Client,
we remove `log4j-kubernetes` from the 2.x release and redirect users to
Fabric8's own `kubernetes-log4j` artifact introduced in fabric8io/kubernetes-client#5718.

The `log4j-kubernetes` lookup depends on:

* the very stable `StrLookup` interface from Log4j Core,
* the evolving set of Kubernetes metadata provided by Kubernetes Client.

Therefore it makes more sense to distribute the lookup together with
Kubernetes Client.

Author: ppkarwasz