[SPARK-24094][SS][MINOR] Change description strings of v2 streaming sources to reflect the change

## What changes were proposed in this pull request?

This makes it easy to understand at runtime which version is running. Great for debugging production issues.

## How was this patch tested?
Not necessary.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #21160 from tdas/SPARK-24094.

external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaMicroBatchReader.scala

@@ -169,7 +169,7 @@ private[kafka010] class KafkaMicroBatchReader(
     kafkaOffsetReader.close()
   }
 
-  override def toString(): String = s"Kafka[$kafkaOffsetReader]"
+  override def toString(): String = s"KafkaV2[$kafkaOffsetReader]"
 
   /**
    * Read initial partition offsets from the checkpoint, or decide the offsets and write them to

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/sources/RateStreamMicroBatchReader.scala

@@ -177,7 +177,7 @@ class RateStreamMicroBatchReader(options: DataSourceOptions, checkpointLocation:
 
   override def stop(): Unit = {}
 
-  override def toString: String = s"MicroBatchRateSource[rowsPerSecond=$rowsPerSecond, " +
+  override def toString: String = s"RateStreamV2[rowsPerSecond=$rowsPerSecond, " +
     s"rampUpTimeSeconds=$rampUpTimeSeconds, " +
     s"numPartitions=${options.get(NUM_PARTITIONS).orElse("default")}"
 }

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/sources/socket.scala

@@ -214,7 +214,7 @@ class TextSocketMicroBatchReader(options: DataSourceOptions) extends MicroBatchR
     }
   }
 
-  override def toString: String = s"TextSocket[host: $host, port: $port]"
+  override def toString: String = s"TextSocketV2[host: $host, port: $port]"
 }
 
 class TextSocketSourceProvider extends DataSourceV2