lcs: only demote sstable from level higher than target one
lcs: only demote sstable from level higher than target one

if we are compacting level 1 into level 2, we only want to demote
a sstable from level 3 or higher.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla