Deprecation of non ISO images

[sp-yduck]

/kind feature

Describe the solution you'd like
cappx uses vnc websocket to download os image into proxmox nodes. As vnc websocket is a bit unstable, it's better to avoid using it. Proxmox officially support ISO type of image and all other images can be converted to ISO easily by using qemu-img convert command or so. then there is no reason to use non iso images at cappx side

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api-provider-proxmox version:
• Proxmox VE version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):