docs: update SECURITY.md, CONTRIBUTING docs, and remove OpenSSF badge placeholder

- SECURITY.md: formalize vulnerability reporting process with GitHub Private
  Security Advisory and email channels, add response SLA (3-day ack, 14-day
  triage, 90-day disclosure), add security response team and disclosure policy
- CONTRIBUTING_EN/CN/JP.md: add test requirements for new functionality
  (30% plugin coverage gate, 50% patch coverage), link security reporting
  to SECURITY.md
- README/README_ZH/README_JP: remove OpenSSF Best Practices badge placeholder
  until passing badge is achieved

Change-Id: Ice19b163c48dab73c903a0b9f4c33ddeff892ebb
Co-developed-by: Kiro <noreply@kiro.dev>
Signed-off-by: EndlessSeeker <1766508902@qq.com>

README_JP.md

@@ -9,7 +9,6 @@
 [![Build Status](https://github.com/alibaba/higress/actions/workflows/build-and-test.yaml/badge.svg?branch=main)](https://github.com/alibaba/higress/actions)
 [![license](https://img.shields.io/github/license/alibaba/higress.svg)](https://www.apache.org/licenses/LICENSE-2.0.html)
 [![CNCF Sandbox](https://img.shields.io/badge/CNCF-Sandbox-30638E?logo=linuxfoundation&logoColor=white)](https://www.cncf.io/projects/)
-[![OpenSSF Best Practices](https://img.shields.io/badge/OpenSSF-Best%20Practices-informational?logo=openssf)](https://www.bestpractices.dev/)
 
 [**公式サイト**](https://higress.cn/)   |
   [**ドキュメント**](https://higress.cn/docs/latest/overview/what-is-higress/)   |