Clean up warnings in Gradle build

See gh-34332

spring-beans/src/testFixtures/java/org/springframework/beans/testfixture/beans/IndexedTestBean.java

@@ -181,6 +181,7 @@ public class IndexedTestBean {
 	}
 
 
+	@SuppressWarnings("serial")
 	public static class IterableMap<K,V> extends LinkedHashMap<K,V> implements Iterable<V> {
 
 		@Override