Do not use 'current' ubuntu image url in template ProxmoxMachine

[sp-yduck]

/kind bug

What steps did you take and what happened:
[A clear and concise description of what the bug is.]

In templates/cluster-template.yaml, image.url is https://cloud-images.ubuntu.com/jammy/current/jammy-server-cloudimg-amd64-disk-kvm.img and image.checksum is 86e996f35732d26cd8b0888c46c4309d4d3b04eb6980378cf82b4d3eb2796549 .
this was working in the past but after image(and ofcourse checksum) updates, this old checksum does not work for this "current" type url. so templates should use imutable image url like https://cloud-images.ubuntu.com/jammy/20230729/*

What did you expect to happen:
templates/cluster-template.yaml works regardless the dates

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):