upgrade: fix wait_for_sstable_upgrade
upgrade: fix wait_for_sstable_upgrade

Step7 of upgrade always fails, it's caused by code issue.
`sstable_versions` is a set, it doesn't support indexing,
this patch converted it to list.

  WARNING > 'set' object does not support indexing

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests