test: update k8s testing versions to 1.15.11, 1.16.8 and 1.17.4
test: update k8s testing versions to 1.15.11, 1.16.8 and 1.17.4

Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

cilium-team/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium-team/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium