THRIFT-5306: Rust code and generated code no longer has clippy warnings (Rust 1.40)

Client: rs

NOTE: Also properly update the min/max supported Rust versions
diff --git a/lib/rs/src/server/threaded.rs b/lib/rs/src/server/threaded.rs
index 8373c46..64bf8bb 100644
--- a/lib/rs/src/server/threaded.rs
+++ b/lib/rs/src/server/threaded.rs
@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+use log::warn;
+
 use std::net::{TcpListener, TcpStream, ToSocketAddrs};
 use std::sync::Arc;
 use threadpool::ThreadPool;