Remove forgotten 1.21 references

README.md

roles/container-engine/cri-o/defaults/main.yml

roles/container-engine/cri-o/vars/amazon.yml

roles/container-engine/cri-o/vars/centos-7.yml

roles/container-engine/cri-o/vars/centos-8.yml

roles/container-engine/cri-o/vars/debian.yml

roles/container-engine/cri-o/vars/fedora.yml

roles/container-engine/cri-o/vars/ubuntu.yml

roles/kubespray-defaults/defaults/main.yaml