Update to containerd v2.1 series

[cwrau]

Is your feature request related to a problem? Please describe.

We'd like to use the "new" image volume feature (https://kubernetes.io/docs/tasks/configure-pod-container/image-volumes/) which needs containerd >=2.1 (https://github.com/containerd/containerd/releases/tag/v2.1.0)

Describe the solution you'd like

CAPI images should be built with containerd v2.1.0 or later.

Describe alternatives you've considered

🤷

Additional context

Since #1708 has been done, I don't expect major problems with this.

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/kind feature

[cwrau]

With k8s 1.36; containerd 1.x will be not be supported anyways (https://kubernetes.io/blog/2025/08/27/kubernetes-v1-34-release/#kubernetes-to-end-containerd-1-x-support-in-v1-36)