Fix private @AfterEach method in IntegrationTestBase

JUnit 5.12+ warns that @AfterEach methods should not be private,
and this will be disallowed in a future release. Changed shutdown()
from private to package-private visibility.

Co-authored-by: torosent <17064840+torosent@users.noreply.github.com>

Author: Copilot

client/src/test/java/com/microsoft/durabletask/IntegrationTestBase.java

@@ -22,7 +22,7 @@ public class IntegrationTestBase {
     private ManagedChannel clientChannel;
 
     @AfterEach
-    private void shutdown() {
+    void shutdown() {
         if (this.server != null) {
             this.server.stop();
             this.server = null;