Cleanup: Ubuntu 20.04 tests (#12301)

* Test: molecule replace ubuntu2004 with ubuntu2204 ubuntu2404

cri-dockerd, adduser and bastion-ssh-config can't run ubuntu2404, maybe needs to check login.

"System is booting up. Unprivileged users are not permitted to log in yet. Please come back later. For technical details, see pam_nologin(8)."

Signed-off-by: ChengHao Yang
<17496418+tico88612@users.noreply.github.com>

* Test: replace ubuntu-2004 with ubuntu-2404

All ubuntu-2004 tests are removed.

Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>

* Docs: update ci.md

Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>

* Docs: update README.md

Remove Ubuntu 20.04 support

Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>

---------

Signed-off-by: ChengHao Yang
Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>

tests/files/ubuntu24-flannel-ha-once.yml

@@ -0,0 +1,21 @@
+---
+# Instance settings
+cloud_image: ubuntu-2404
+mode: ha
+
+# Kubespray settings
+kubeadm_certificate_key: 3998c58db6497dd17d909394e62d515368c06ec617710d02edea31c06d741085
+kube_proxy_mode: iptables
+kube_network_plugin: flannel
+helm_enabled: true
+kubernetes_audit: true
+etcd_events_cluster_enabled: true
+local_volume_provisioner_enabled: true
+kube_encrypt_secret_data: true
+ingress_nginx_enabled: true
+cert_manager_enabled: true
+# Disable as health checks are still unstable and slow to respond.
+metrics_server_enabled: false
+metrics_server_kubelet_insecure_tls: true
+kube_token_auth: true
+enable_nodelocaldns: false