No longer works or required for Ubuntu >= 23.10

[dannf]

Starting with 23.10 Ubuntu has changed where GRUB/shim downloads are located. But, also starting with 23.10, they provide pre-built netboot tarballs, e.g.:

https://releases.ubuntu.com/23.10/ubuntu-23.10-netboot-amd64.tar.gz
https://cdimage.ubuntu.com/releases/mantic/release/ubuntu-23.10-netboot-arm64.tar.gz

And there are unpacked versions, such as:
https://cdimage.ubuntu.com/releases/mantic/release/netboot/arm64/

Should we deprecate this tool for newer releases? I wonder if there's any value in teaching it to download these tarballs and allow the configuration options (--extra-args, --url, etc) work on top of them.