docs(security): direct vulnerability reports to GitHub security advisories (#41848)

## Summary
- Replace the `security@appsmith.com` mailing list with a link to
GitHub's private vulnerability reporting in `SECURITY.md`.

## Test plan
- [ ] Confirm the link in the rendered `SECURITY.md` opens the
new-advisory form on `appsmithorg/appsmith`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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## Summary by CodeRabbit

* **Documentation**
* Updated security vulnerability reporting instructions to direct
reporters to GitHub's private vulnerability reporting form for improved
security incident handling.

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Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>