mboersma: Default Ubuntu to 20.04 for new k8s versions

JobBuild
1508
07-30 01:25
pull-cluster-api-provider-azure-build 1420918468043083776
pull-cluster-api-provider-azure-e2e 1420918468093415424
pull-cluster-api-provider-azure-e2e-windows 1420918468118581248
pull-cluster-api-provider-azure-test 1420918468009529344
pull-cluster-api-provider-azure-verify 1420918468147941376