docs: point Japanese security link to Japanese policy

[kitsuyui]

What

Updated the Japanese CONTRIBUTING security link to SECURITY.ja.md.

Why

Keeps the English and Japanese community-health docs aligned for Japanese readers.

Checks

• git diff --check
• npx --yes markdownlint-cli2 CONTRIBUTING.ja.md SECURITY.ja.md
• typos CONTRIBUTING.ja.md SECURITY.ja.md
• npx --yes markdown-link-check CONTRIBUTING.ja.md -q
• check-pr-collateral.py --title 'docs: point Japanese security link to Japanese policy' --format md
• Full-repo markdownlint and cspell still fail on pre-existing unrelated baseline issues in CODE_OF_CONDUCT.md, the PR template line length, and kitsuyui.