Spec Text:
https://tc39.es/proposal-temporal/#sec-temporal.zoneddatetime.prototype.withplaindate

Bug: v8:11544
Change-Id: I8d7099906c04eae0e29ff0dabd48b736a07305d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3692432
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80984}