[SPARK-37860][UI] Fix taskindex in the stage page task event timeline

### What changes were proposed in this pull request?
This reverts commit 450b415.

### Why are the changes needed?
In apache#32888, shahidki31 change taskInfo.index to taskInfo.taskId. However, we generally use `index.attempt` or `taskId` to distinguish tasks within a stage, not `taskId.attempt`.
Thus apache#32888 was a wrong fix issue, we should revert it.

### Does this PR introduce _any_ user-facing change?
no

### How was this patch tested?
origin test suites

Closes apache#35160 from stczwd/SPARK-37860.

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

Author: stczwd

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -355,7 +355,7 @@ private[ui] class StagePage(parent: StagesTab, store: AppStatusStore) extends We
                |'content': '<div class="task-assignment-timeline-content"
                  |data-toggle="tooltip" data-placement="top"
                  |data-html="true" data-container="body"
-                 |data-title="${s"Task " + taskInfo.taskId + " (attempt " + attempt + ")"}<br>
+                 |data-title="${s"Task " + index + " (attempt " + attempt + ")"}<br>
                  |Status: ${taskInfo.status}<br>
                  |Launch Time: ${UIUtils.formatDate(new Date(launchTime))}
                  |${