Use correct nullable annotations in smoke tests

See gh-46926

Author: mhalbritter

smoke-test/spring-boot-smoke-test-web-thymeleaf/src/main/java/smoketest/web/thymeleaf/Message.java

@@ -18,8 +18,8 @@
 
 import java.util.Calendar;
 
-import jakarta.annotation.Nullable;
 import jakarta.validation.constraints.NotEmpty;
+import org.jspecify.annotations.Nullable;
 
 public class Message {
 

smoke-test/spring-boot-smoke-test-websocket-jetty/src/main/java/smoketest/websocket/jetty/snake/SnakeTimer.java

@@ -24,9 +24,9 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 
-import jakarta.annotation.Nullable;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Sets up the timer for the multiplayer snake game WebSocket example.