[SPARK-26318][SQL] Deprecate Row.merge

## What changes were proposed in this pull request?
Deprecate Row.merge

## How was this patch tested?
N/A

Closes #23271 from KyleLi1985/master.

Authored-by: 李亮 <liang.li.work@outlook.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

sql/catalyst/src/main/scala/org/apache/spark/sql/Row.scala

@@ -57,6 +57,7 @@ object Row {
   /**
    * Merge multiple rows into a single row, one after another.
    */
+  @deprecated("This method is deprecated and will be removed in future versions.", "3.0.0")
   def merge(rows: Row*): Row = {
     // TODO: Improve the performance of this if used in performance critical part.
     new GenericRow(rows.flatMap(_.toSeq).toArray)