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.md

@@ -12,7 +12,6 @@
 [![license](https://img.shields.io/github/license/alibaba/higress.svg)](https://www.apache.org/licenses/LICENSE-2.0.html)
 [![discord](https://img.shields.io/discord/1364956090566971515?color=5865F2&label=discord&labelColor=black&logo=discord&logoColor=white&style=flat-square)](https://discord.gg/tSbww9VDaM)
 [![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/)
 
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