Replace usage of gdisk with sfdisk

[Conan-Kudo]

We currently use gdisk for GPT disks due to a legacy in which GPT was only supported by gdisk, but fdisk and sfdisk from util-linux has supported GPT in addition to MBR since 2013 with util-linux 2.23. We no longer support any distribution that doesn't have a version of fdisk that doesn't support GPT, so let's move to sfdisk and drop the gdisk/gptfdisk dependency.

[Conan-Kudo]

Also note, gdisk not available in RHEL 10 either, and RHEL users are expected to use fdisk/sfdisk instead.

[Conan-Kudo]

We may want to also migrate our fdisk usage to sfdisk too, to simplify how we interact with partitioning.

[github-actions]

This issue is stale because it has been open 60 days with no activity. stale label was set.

[Conan-Kudo]

This is still planned to be done.