service: storage_service: leave Raft group 0 before `stop_transport` in `decommission`
service: storage_service: leave Raft group 0 before `stop_transport` in `decommission`

Leaving group 0 in `decommission` would previously fail with RPC
exception because it happened after messaging service was shutdown.

Fixes #9845.
Message-Id: <e26f7c3f605cca743bb90b2e6b62064bedfb3ebb@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla