Fix remaining compiler warnings

Fix remaining Java compiler warnings, mainly around missing
generics or deprecated code.

Also add the `-Werror` compiler option to ensure that any future
warnings will fail the build.

Issue: SPR-11064

spring-messaging/src/main/java/org/springframework/messaging/core/DestinationResolver.java

@@ -13,20 +13,22 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.messaging.core;
 
-
 /**
+ * Strategy interface that resolves a given name to a destination.
+ *
  * @author Mark Fisher
  * @since 4.0
  */
 public interface DestinationResolver<D> {
 
-
 	/**
-	 * @param name
-	 * @return
-	 * @throws DestinationResolutionException
+	 * Resolve the given {@code name} to a destination.
+	 * @param name the name to resolve
+	 * @return the destination (must not be {@code null})
+	 * @throws DestinationResolutionException if the name cannot be resolved
 	 */
 	D resolveDestination(String name) throws DestinationResolutionException;
 

spring-messaging/src/main/java/org/springframework/messaging/simp/annotation/support/SendToMethodReturnValueHandler.java

@@ -140,7 +140,7 @@ public class SendToMethodReturnValueHandler implements HandlerMethodReturnValueH
 		else {
 			SendTo sendTo = returnType.getMethodAnnotation(SendTo.class);
 			String[] destinations = getTargetDestinations(sendTo, inputHeaders, this.defaultDestinationPrefix);
-			for (String destination : getTargetDestinations(sendTo, inputHeaders, this.defaultDestinationPrefix)) {
+			for (String destination : destinations) {
 				this.messagingTemplate.convertAndSend(destination, returnValue, postProcessor);
 			}
 		}

spring-messaging/src/main/java/org/springframework/messaging/simp/handler/UserDestinationMessageHandler.java

@@ -16,20 +16,17 @@
 
 package org.springframework.messaging.simp.handler;
 
+import java.util.Set;
+
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHandler;
 import org.springframework.messaging.MessagingException;
 import org.springframework.messaging.core.MessageSendingOperations;
-import org.springframework.messaging.simp.SimpMessageHeaderAccessor;
-import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 
-import java.util.Set;
-
-
 /**
  * Provides support for messages sent to "user" destinations, translating the
  * destination to one or more user-specific destination(s) and then sending message(s)