Add SECURITY.md with security policy

[anjali1521]

Related Issue

Closes #1047

Summary

This PR updates the Security Policy to provide clearer information about supported versions and the process for reporting vulnerabilities.

Changes Made

• Clarified supported versions table with ✅ and ❌ indicators
• Added detailed instructions for reporting vulnerabilities
• Defined response time expectations and confidentiality guidelines
• Outlined resolution process for accepted and declined reports

Testing

• Verified formatting renders correctly in GitHub markdown preview
• Checked table alignment and emoji rendering
• Reviewed links to ensure they point to valid GitHub documentation

Screenshots

N/A (no UI changes)

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (SECURITY.md)