[SPARK-25202][SQL][FOLLOW-UP] Keep the old parameter name 'pattern' at split in Scala API

### What changes were proposed in this pull request?

To address the concern pointed out in #22227. This will make `split` source-compatible by removing minimal cosmetic changes.

### Why are the changes needed?

For source compatibility.

### Does this PR introduce any user-facing change?

No (it will prevent potential user-facing change from the original PR)

### How was this patch tested?

Unittest was changed (in order for us to detect that source compatibility easily).

Closes #27756 from HyukjinKwon/SPARK-25202.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
(cherry picked from commit 3956e95)
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: HyukjinKwon

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -2460,25 +2460,25 @@ object functions {
   def soundex(e: Column): Column = withExpr { SoundEx(e.expr) }
 
   /**
-   * Splits str around matches of the given regex.
+   * Splits str around matches of the given pattern.
    *
    * @param str a string expression to split
-   * @param regex a string representing a regular expression. The regex string should be
-   *              a Java regular expression.
+   * @param pattern a string representing a regular expression. The regex string should be
+   *                a Java regular expression.
    *
    * @group string_funcs
    * @since 1.5.0
    */
-  def split(str: Column, regex: String): Column = withExpr {
-    StringSplit(str.expr, Literal(regex), Literal(-1))
+  def split(str: Column, pattern: String): Column = withExpr {
+    StringSplit(str.expr, Literal(pattern), Literal(-1))
   }
 
   /**
-   * Splits str around matches of the given regex.
+   * Splits str around matches of the given pattern.
    *
    * @param str a string expression to split
-   * @param regex a string representing a regular expression. The regex string should be
-   *              a Java regular expression.
+   * @param pattern a string representing a regular expression. The regex string should be
+   *                a Java regular expression.
    * @param limit an integer expression which controls the number of times the regex is applied.
    *        <ul>
    *          <li>limit greater than 0: The resulting array's length will not be more than limit,
@@ -2491,8 +2491,8 @@ object functions {
    * @group string_funcs
    * @since 3.0.0
    */
-  def split(str: Column, regex: String, limit: Int): Column = withExpr {
-    StringSplit(str.expr, Literal(regex), Literal(limit))
+  def split(str: Column, pattern: String, limit: Int): Column = withExpr {
+    StringSplit(str.expr, Literal(pattern), Literal(limit))
   }
 
   /**

sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala

@@ -196,7 +196,7 @@ class DataFrameSuite extends QueryTest
   test("explode on output of array-valued function") {
     val df = Seq(("1,2"), ("4"), ("7,8,9")).toDF("csv")
     checkAnswer(
-      df.select(explode(split($"csv", ","))),
+      df.select(explode(split($"csv", pattern = ","))),
       Row("1") :: Row("2") :: Row("4") :: Row("7") :: Row("8") :: Row("9") :: Nil)
   }