test: update k8s tested versions
test: update k8s tested versions

Update to 1.18.17, 1.19.9 and 1.20.5

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

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