chore(bootstrap): upgrade kubespray to release-2.28 and kubernetes to v1.32.13

- Bump Kubespray baseline to release-2.28 (last release supporting Ubuntu 20.04)
- Upgrade Kubernetes control plane and node to v1.32.13
- Configure single-node cluster settings (drain_nodes: false)
- Add patch for Kubespray v1beta4 kubeadm preflight error handling
- Update init.sh, AGENTS.md, and README.md with two-phase upgrade instructions

bootstrap/inventory/prod/group_vars/k8s_cluster/k8s-cluster.yml

@@ -27,7 +27,17 @@ kube_kubeadm_apiserver_extra_args:
 kube_api_anonymous_auth: true
 
 ## Change this to use another Kubernetes version, e.g. a current beta release
-kube_version: v1.31.9
+kube_version: 1.32.13
+
+# Single-node cluster settings: prevent draining or cordoning the only node
+drain_nodes: false
+upgrade_node_always_cordon: false
+upgrade_node_fail_if_drain_fails: false
+kubeadm_ignore_preflight_errors:
+  - ImagePull
+  - SystemVerification
+  - FileContent--proc-sys-net-bridge-bridge-nf-call-iptables
+  - CreateJob
 
 # kubernetes image repo define
 #kube_image_repo: "k8s.gcr.io"