[RFE] Upgrade systemd 255 to systemd 256

[ader1990]

Current situation

Currently, Flatcar uses systemd 255 and the usual historical cadence of a systemd version is one every 6 months, and for Flatcar is of around 1-2 years.

Opening this issue to track possible blockers for the 255 -> 256 upgrade early, as there might be the need of structural changes to the Flatcar layout and how to's, as systemd is a cornerstone critical package for Flatcar, along with the kernel, ignition and containerd (k8s).

Also, the mutable sysext / confext patches for 255 that we currently have already borrow functionality from 256.

Ideal future situation

Upgrade to systemd version 256.

Current blockers:

• [ ] ignition-setup.service fails to run because /usr is now read only
• [ ] Systemd 256 marks the system as tainted is /usr/bin is not symlinked to /usr/sbin (systemd: system is tainted: unmerged bin)