[SPARK-30566][BUILD] Iterator doesn't refer outer identifier named "iterator" properly in Scala 2.13

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

Renamed an identifier `iterator` to `iter` to avoid compile error with Scala 2.13.

### Why are the changes needed?

As of Scala 2.13, scala.collection.Iterator has "iterator" method so if an inner class of Iterator means to refer an outer identifier named "iterator", it does not work as we think.
I listed source files that can be affected by that change by `find . -name "*.scala" -exec grep -El "new .*Iterator\[.* +{"  {} \;`
As far as I confirmed util.Utils` is affected.

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

No.

### How was this patch tested?

Existing tests.

Closes #27275 from sarutak/fix-iterator-for-2.13.

Authored-by: Kousuke Saruta <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: sarutak

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -1812,14 +1812,14 @@ private[spark] object Utils extends Logging {
    * Generate a zipWithIndex iterator, avoid index value overflowing problem
    * in scala's zipWithIndex
    */
-  def getIteratorZipWithIndex[T](iterator: Iterator[T], startIndex: Long): Iterator[(T, Long)] = {
+  def getIteratorZipWithIndex[T](iter: Iterator[T], startIndex: Long): Iterator[(T, Long)] = {
     new Iterator[(T, Long)] {
       require(startIndex >= 0, "startIndex should be >= 0.")
       var index: Long = startIndex - 1L
-      def hasNext: Boolean = iterator.hasNext
+      def hasNext: Boolean = iter.hasNext
       def next(): (T, Long) = {
         index += 1L
-        (iterator.next(), index)
+        (iter.next(), index)
       }
     }
   }