[SPARK-45641][UI] Display the application start time on AllJobsPage

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

This PR adds content for the application start time on AllJobsPage.

### Why are the changes needed?

The application start time currently remains in a long value form, which is not convenient for users to read on the environment tab. It's also useful on the AllJobsPage, such as identifying a long pause before job 0 gets scheduled.

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

yes, UI updates

### How was this patch tested?

locally tested

![Screenshot 2023-10-24 at 13 44 19](https://github.com/apache/spark/assets/8326978/d0a7ccf5-43e5-4780-b937-e26ca9a91d36)

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

no

Closes apache#43495 from yaooqinn/SPARK-45641.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

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

@@ -277,7 +277,8 @@ private[ui] class AllJobsPage(parent: JobsTab, store: AppStatusStore) extends We
 
   def render(request: HttpServletRequest): Seq[Node] = {
     val appInfo = store.applicationInfo()
-    val startTime = appInfo.attempts.head.startTime.getTime()
+    val startDate = appInfo.attempts.head.startTime
+    val startTime = startDate.getTime()
     val endTime = appInfo.attempts.head.endTime.getTime()
 
     val activeJobs = new ListBuffer[v1.JobData]()
@@ -327,6 +328,10 @@ private[ui] class AllJobsPage(parent: JobsTab, store: AppStatusStore) extends We
             <strong>User:</strong>
             {parent.getSparkUser}
           </li>
+          <li>
+            <strong>Started At:</strong>
+            {UIUtils.formatDate(startDate)}
+          </li>
           <li>
             <strong>Total Uptime:</strong>
             {