Spec Text: https://tc39.es/proposal-temporal/#sec-temporal.plaindate.prototype.toplaindatetime

Bug: v8:11544
Change-Id: Ie86a5d8a2f1206a44d0e74638392ae26e0f8e900
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3534454Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80707}