sbueringer: 📖 update supported Kubernetes versions

JobBuild
5012
07-23 18:08
pull-cluster-api-apidiff-main 1420632467219419136 1420458955057926144 1418634173001764864
pull-cluster-api-build-main 1420632467185864704 1420458955020177408 1418634172964016128
pull-cluster-api-test-main 1420632467290722304 1420458955150200832 1418634173064679424
pull-cluster-api-verify 1420632467248779264 1420458955108257792 1418634173039513600