update version to 2.3.1

Author: cindy-peng

google-cloud-datastore/src/main/java/com/google/cloud/datastore/testing/LocalDatastoreHelper.java

@@ -59,7 +59,7 @@ public class LocalDatastoreHelper extends BaseEmulatorHelper<DatastoreOptions> {
   private static final String GCLOUD_CMD_TEXT = "gcloud beta emulators datastore start";
   private static final String GCLOUD_CMD_PORT_FLAG = "--host-port=";
   private static final String VERSION_PREFIX = "cloud-datastore-emulator ";
-  private static final String MIN_VERSION = "2.2.1";
+  private static final String MIN_VERSION = "2.3.1";
 
   // Downloadable emulator settings
   private static final String BIN_NAME = "cloud-datastore-emulator/cloud_datastore_emulator";