Turning on composefs breaks pivot from old bootimages

Updated description:

Any bootimage with a libostree older than v2023.4 will be too old to know how to create composefs images when upgrading. And so once we upgrade, if composefs = yes in prepare-root.conf, ostree-prepare-root in the new initramfs will fail because it sees that composefs should be enabled and yet no composefs image exists. We currently set composefs = maybe in prepare-root.conf because of this: https://github.com/coreos/rhel-coreos-config/blob/2c3f8527babb935d8d71b505bc764dbab8163b69/manifest-el9-shared.yaml#L28-L34

This ticket tracks being able to switch it back to yes.

Original description follows.

Attached the full secondboot journal.
journal.txt

The specific error is:

Dec 09 17:13:23 localhost systemd[1]: Mounted /sysroot.
Dec 09 17:13:23 localhost systemd[1]: Starting OSTree Prepare OS/...
Dec 09 17:13:23 localhost ostree-prepare-root[675]: Resolved OSTree target to: /sysroot/ostree/deploy/fedora-coreos/deploy/8bd789c8d973d9910e8cc1fc477dd20ac5e975e0663085eeb06a95bfcdba5199.0
Dec 09 17:13:23 localhost ostree-prepare-root[675]: sysroot.readonly configuration value: 1 (fs writable: 1)
Dec 09 17:13:23 localhost ostree-prepare-root[675]: ostree-prepare-root: composefs: failed to mount: No such file or directory
Dec 09 17:13:23 localhost systemd[1]: ostree-prepare-root.service: Main process exited, code=exited, status=1/FAILURE
Dec 09 17:13:23 localhost systemd[1]: ostree-prepare-root.service: Failed with result 'exit-code'.
Dec 09 17:13:23 localhost systemd[1]: Failed to start OSTree Prepare OS/.

Read manifest-el9-shared.yaml at the linked revision, then trace how prepare-root.conf sets composefs and how ostree-prepare-root handles bootimages built with libostree older than v2023.4. Confirm the compatibility path that allows composefs to be switched from maybe to yes without the reported mount failure, using the attached secondboot journal as the failure reference.