Remove linear RAID as an option

[marmijo]

md-linear has been deprecated for 2.5 years and was finally removed from the kernel in v6.8 [1]. The kola test for this kind of RAID was removed from FCOS and RHCOS in coreos/fedora-coreos-config#2830.

We should remove linear RAID as a configuration option from Ignition as well.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v6.8-rc1&id=849d18e27be9a1253f2318cb4549cc857219d991

[jlebon]

We can't remove this while there are distros shipping Ignition with users that might be relying on it. We also don't currently have strict policies on the minimum kernel version supported in Ignition.

Probably not worth trying to be aggressive with this. I would just leave it in at least until all the major consumers we know of are on v6.8.