[SPARK-46611][CORE][FOLLOWUP] Correct new use of DateTimeFormat.withZone in UIUtils

[srowen]

What changes were proposed in this pull request?

Simplify code and fix new use of DateTimeFormat.withZone introduced in #44613 ; need to use the new object copy this creates.

Why are the changes needed?

Describing on mailing list from Janda Martin:

DateTimeFormatter is thread-safe and immutable according to JavaDoc so method DateTimeFormatter::withZone returns new instance when zone is changed.

Following code has no effect:
  val oldTimezones = (batchTimeFormat.getZone, batchTimeFormatWithMilliseconds.getZone)
  if (timezone != null) {
      val zoneId = timezone.toZoneId
      batchTimeFormat.withZone(zoneId)
      batchTimeFormatWithMilliseconds.withZone(zoneId)
    }

Suggested fix:
introduce local variables for "batchTimeFormat" and "batchTimeFormatWithMilliseconds" and remove "oldTimezones" and "finally" block.

Does this PR introduce any user-facing change?

Unlikely, the path in question is apparently test-only

How was this patch tested?

Existing tests

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

No

[HyukjinKwon]

Build https://github.com/srowen/spark/actions/runs/7444863091

[beliefer]

LGTM if tests passed.

[HyukjinKwon]

Merged to master.