CI: add Ubuntu 26.04 support and coverage (#13443)

* image-builder: add Ubuntu 26.04 image

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

* CI: add Ubuntu 26.04 support

Add an Ubuntu 26.04 CI job (ubuntu26-calico-all-in-one) to the main PR
matrix, OS-specific node vars, and update the coverage docs.

The VM image is published as quay.io/kubespray/vm-ubuntu-2604.

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

---------

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

tests/files/ubuntu26-calico-all-in-one.yml

@@ -0,0 +1,27 @@
+---
+# Instance settings
+cloud_image: ubuntu-2604
+mode: all-in-one
+vm_memory: 3072
+
+# Kubespray settings
+auto_renew_certificates: true
+
+# Currently ipvs not available on KVM: https://packages.ubuntu.com/search?suite=resolute&arch=amd64&mode=exactfilename&searchon=contents&keywords=ip_vs_sh.ko
+kube_proxy_mode: iptables
+enable_nodelocaldns: false
+
+containerd_registries_mirrors:
+  - prefix: docker.io
+    mirrors:
+      - host: https://mirror.gcr.io
+        capabilities: ["pull", "resolve"]
+        skip_verify: false
+  - prefix: 172.19.16.11:5000
+    mirrors:
+      - host: http://172.19.16.11:5000
+        capabilities: ["pull", "resolve", "push"]
+        skip_verify: true
+
+kube_reserved: true
+system_reserved: true