bump kuberntes-client to latest kubernetes-client 6.12.0

[adietish]

redhat-developer/intellij-kubernetes#726 in intellij-kubernetes will most likely require a bump in a fixed kubernetes-client (6.11.1).

[adietish]

6.11.0 did not fix fabric8io/kubernetes-client#5817: https://github.com/fabric8io/kubernetes-client/releases/tag/v6.11.0
Targeting 6.12.0 then.