Remove pinning from outdated k0s-imported OCI images

[twz123]

When k0s imports OCI images into containerd, it pins them to prevent them from being garbage collected. If the OCI images are replaced regularly over time, e.g. if an airgapped cluster is regularly upgraded, these images will pile up in containerd's image filesystem, potentially leading to disk pressure at some point.

K0s should be smart about this. It should unpin imported images that are no longer in the import folder. This will allow containerd to remove any old images once they are no longer in use.