Use okhttp-api plugin instead of bundling okhttp from kubernetes-client

[Vlatombe]

This also moves to okhttp 4.x since this is what the plugin provides.

https://github.com/fabric8io/kubernetes-client/blob/master/doc/KubernetesClientWithIPv6Clusters.md

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[Vlatombe]

Test works fine in jenkinsci/kubernetes-plugin#1330.