docs: clarify Kubernetes support window (#13216)

Document the rolling three-minor Kubernetes support window and derive the README minimum version from kube_version_min_required.

Signed-off-by: Kay Yan <kay.yan@daocloud.io>

scripts/propagate_ansible_variables.yml

@@ -20,6 +20,12 @@
       marker: '<!-- {mark} ANSIBLE MANAGED BLOCK -->'
       block: "\n{{ lookup('ansible.builtin.template', 'readme_versions.md.j2') }}\n\n"
       path: ../README.md
+  - name: Render minimum Kubernetes version in README.md
+    lineinfile:
+      path: ../README.md
+      regexp: '^- \*\*Minimum required version of Kubernetes is v.*\*\*$'
+      line: "- **Minimum required version of Kubernetes is v{{ kube_version_min_required }}**"
+      insertafter: '^## Requirements$'
   - name: Render Dockerfiles
     template:
       src: "{{ item }}.j2"