composefs backend lacks garbage collection for /boot and sysroot

[bketelsen]

Observed:
After running a few updates, /boot/EFI/Linux fills up and prevents bootc update from finishing, which leaves the system in a bad state.

/sysroot/composefs/images also doesn't appear to be pruned after updates.

Expected:
After an update, any un-reachable kernels and composefs images would be removed, freeing disk space. Un-reachable probably means that it's not listed in a boot loader entry somewhere, but there may be more nuance.

Thanks for making the composefs backend, we're having tons of fun with Debian :)

[cgwalters]

cc containers/composefs-rs#196