Migrate from gdisk (sgdisk) to fdisk (sfdisk)

[travier]

Describe the enhancement

There is a push from the Enterprise Linux side of things to migrate off gdisk and unify on the fdisk utilities. This would also help us unify on a single tool for partitioning as we currently build the disk images with fdisk utilities via osbuild but use gdisk on deployed systems.

Some related issues from this discrepancy:

• LUKS on 512e disk breaks ignition-ostree-growfs.service #1384
• Different partition layout after re-provisioning between version 39 and 40 #1745

This will require some work in:

• Ignition: Support sfdisk backend for partitioning ignition#1926
• fedora-coreos-config: this issue

All sgdisk mentions in the f-c-c: https://github.com/search?q=repo%3Acoreos%2Ffedora-coreos-config%20sgdisk&type=code

We will hav to watch out for regressions related to partitioning.

System details

N/A

Additional information

Initial work in coreos/fedora-coreos-config#3015

[dustymabe]

WIP work for this in Ignition upstream: coreos/ignition#1929

[marmijo]

This was discussed in the Fedora CoreOS community meeting on 2025-03-05:

We're seeking a volunteer to take on the Ignition gdisk->fdisk migration work. 
Let us know if you're interested!

[dustymabe]

related: dropping sgdisk from cloud-init

https://src.fedoraproject.org/rpms/cloud-init/pull-request/51