Also change some AO's return type from MaybeHandle to Handle
and call ToHandleChecked() on the right spot.

Spec Text:
https://tc39.es/proposal-temporal/#sec-temporal.zoneddatetime.prototype.toinstant
https://tc39.es/proposal-temporal/#sec-temporal.zoneddatetime.prototype.toplaindate
https://tc39.es/proposal-temporal/#sec-temporal.zoneddatetime.prototype.toplaintime
https://tc39.es/proposal-temporal/#sec-temporal.zoneddatetime.prototype.toplaindatetime

Bug: v8:11544
Change-Id: I140b6a7053d00837bc48fb70980d69778715c0c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3385603
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80788}