cls/rbd: mark cls_rbd_snap as forward incompatible since nautilus

cls/rbd: mark cls_rbd_snap as forward incompatible since nautilus

ceph/ceph@32b14ed1 broke backward compatibility of cls_rbd_snap's on-disk
format by removing decoding of the "parent" field. When re-encoding
instances, this field is now lost even if originally non-empty.

This breaking change should have been caught by readable.sh tests, but
the tests passed because they reused the same instance for re-encoding,
preserving undecoded fields by only overwriting decoded ones.

An upcoming change in ceph/ceph will allocate fresh instances for each
decode operation instead of reusing existing ones. This will expose the
compatibility break and cause test failures.

Mark cls_rbd_snap as forward incompatible starting from v15 (earliest
version with archived corpus) to properly handle this breaking change
and prepare for the improved testing approach.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>