revert: go back to streaming mode and update docs instead

Author: operte

lightgbm/src/main/scala/com/microsoft/azure/synapse/ml/lightgbm/params/LightGBMParams.scala

@@ -91,8 +91,8 @@ trait LightGBMExecutionParams extends Wrappable {
 
   val dataTransferMode = new Param[String](this, "dataTransferMode",
     "Specify how SynapseML transfers data from Spark to LightGBM.  " +
-      "Values can be streaming, bulk. Default is bulk, which is the legacy mode.")
-  setDefault(dataTransferMode -> LightGBMConstants.BulkDataTransferMode)
+      "Values can be streaming, bulk. Default is streaming.")
+  setDefault(dataTransferMode -> LightGBMConstants.StreamingDataTransferMode)
   def getDataTransferMode: String = $(dataTransferMode)
   def setDataTransferMode(value: String): this.type = set(dataTransferMode, value)