[SPARK-18555][MINOR][SQL] Fix the @since tag when backporting from 2.2 branch into 2.1 branch

## What changes were proposed in this pull request?

Fix the since tag when backporting critical bugs (SPARK-18555) from 2.2 branch into 2.1 branch.

## How was this patch tested?

N/A

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: DB Tsai <dbtsai@dbtsai.com>

Closes #17600 from dbtsai/branch-2.1.

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

@@ -128,7 +128,7 @@ final class DataFrameNaFunctions private[sql](df: DataFrame) {
   /**
    * Returns a new `DataFrame` that replaces null or NaN values in numeric columns with `value`.
    *
-   * @since 2.2.0
+   * @since 2.1.1
    */
   def fill(value: Long): DataFrame = fill(value, df.columns)
 
@@ -149,7 +149,7 @@ final class DataFrameNaFunctions private[sql](df: DataFrame) {
    * Returns a new `DataFrame` that replaces null or NaN values in specified numeric columns.
    * If a specified column is not a numeric column, it is ignored.
    *
-   * @since 2.2.0
+   * @since 2.1.1
    */
   def fill(value: Long, cols: Array[String]): DataFrame = fill(value, cols.toSeq)
 
@@ -165,7 +165,7 @@ final class DataFrameNaFunctions private[sql](df: DataFrame) {
    * (Scala-specific) Returns a new `DataFrame` that replaces null or NaN values in specified
    * numeric columns. If a specified column is not a numeric column, it is ignored.
    *
-   * @since 2.2.0
+   * @since 2.1.1
    */
   def fill(value: Long, cols: Seq[String]): DataFrame = fillValue(value, cols)