deprecate CLI flag wconfig

follow up issue of https://github.com/aws/eks-anywhere/issues/5905
The bug fixed by https://github.com/aws/eks-anywhere/pull/6666

**What would you like to be added**:
use mgmt kubeconfig to get wl kubeconfig instead of asking customer to provide one
https://cluster-api.sigs.k8s.io/clusterctl/commands/get-kubeconfig

**Why is this needed**:

Start by reading follow-up issue 5905, the bug-fixing pull request 6666, and the referenced clusterctl get-kubeconfig entry point. Trace how the CLI flag wconfig currently obtains the workload-cluster kubeconfig; done means using the management kubeconfig instead of asking the customer to provide one and deprecating the flag.

Written by the indexing model from the issue text.