Declare serialVersionUID explicitly

Author: marcphilipp

junit-platform-engine/src/main/java/org/junit/platform/engine/support/hierarchical/ForkJoinPoolHierarchicalTestExecutorService.java

@@ -16,6 +16,7 @@
 import static org.junit.platform.engine.support.hierarchical.Node.ExecutionMode.CONCURRENT;
 import static org.junit.platform.engine.support.hierarchical.Node.ExecutionMode.SAME_THREAD;
 
+import java.io.Serial;
 import java.lang.Thread.UncaughtExceptionHandler;
 import java.lang.reflect.Constructor;
 import java.util.ArrayDeque;
@@ -223,6 +224,9 @@ public void close() {
 	@SuppressWarnings({ "serial", "RedundantSuppression" })
 	class ExclusiveTask extends ForkJoinTask<Void> {
 
+		@Serial
+		private static final long serialVersionUID = 1;
+
 		private final TestTask testTask;
 
 		ExclusiveTask(TestTask testTask) {