[Temporal] Sync to PR2248 to use GetTemporalUnit

https://github.com/tc39/proposal-temporal/pull/2248

Bug: v8:11544
Change-Id: I02612030f626adf2b163dca96c7602efe6adef4a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3712044Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81503}