Fix NPE when gracefully shutting down netty

Closes gh-35264

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/netty/GracefulShutdown.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -78,7 +78,7 @@ final class GracefulShutdown {
 			while (!this.shuttingDown) {
 				sleep(50);
 			}
-			this.shutdownThread.interrupt();
+			shutdownThread.interrupt();
 		}
 	}