[SPARK-48565][UI] Fix thread dump display in UI

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

Thread dump display in UI is not pretty as before, this is side-effect introduced by SPARK-44863

### Why are the changes needed?

Restore thread dump display in UI.

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

Yes, it only affects UI display.

### How was this patch tested?

Current master:
<img width="1545" alt="master-branch" src="https://github.com/apache/spark/assets/26535726/5c6fd770-467f-481c-a635-2855a2853633">

With this patch applied:
<img width="1542" alt="Xnip2024-06-07_20-00-38" src="https://github.com/apache/spark/assets/26535726/3998c2aa-671f-4921-8444-b7bca8667202">

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #46916 from pan3793/SPARK-48565.

Authored-by: Cheng Pan <[email protected]>
Signed-off-by: Kent Yao <[email protected]>

Author: pan3793

core/src/main/scala/org/apache/spark/status/api/v1/api.scala

@@ -510,7 +510,7 @@ case class StackTrace(elems: Seq[String]) {
   override def toString: String = elems.mkString
 
   def html: NodeSeq = {
-    val withNewLine = elems.foldLeft(NodeSeq.Empty) { (acc, elem) =>
+    val withNewLine = elems.map(_.stripLineEnd).foldLeft(NodeSeq.Empty) { (acc, elem) =>
       if (acc.isEmpty) {
         acc :+ Text(elem)
       } else {