fix: remove invalid null last_update front matter causing Docusaurus build failure

[Copilot]

Docusaurus v3.10.1 rejects last_update: null in front matter, causing the entire build to fail when loading doc metadata.

Root Cause

147 files in docs/ had bare last_update: keys (no value), which YAML parses as null. Docusaurus validates this field strictly — it must be either an author string or a date string/object.

Before:

---
title: JHipsterドメイン言語(JDL) - アプリケーション
slug: /jdl/applications
last_update:
date: 2026-05-23T12:00:00-00:00
---

After:

---
title: JHipsterドメイン言語(JDL) - アプリケーション
slug: /jdl/applications
date: 2026-05-23T12:00:00-00:00
---

Changes

• Removed empty last_update: lines from all 147 affected .mdx/.md files under docs/