Change Japanese locale calendar format from Japanese era to Gregorian

[miyaichi]

Summary

Changed the calendar format for the Japanese locale from Japanese era (Reiwa) to Gregorian calendar
(Western calendar) in the Docusaurus configuration.

Changes

• Updated calendar setting from "japanese" to "gregory" for the Japanese locale in
  docusaurus.config.js

Impact

• Last update dates in Japanese documentation will now display in Western calendar format (e.g.,
  "2025年3月26日") instead of Japanese era format (e.g., "令和7年3月26日")
• This makes dates more universally readable and consistent with international standards

Testing

• Start the development server and verify that dates in Japanese documentation pages display in Western
  calendar format