docs: add SECURITY.md with private vulnerability reporting policy - #43

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/add-to-project@244f685bbc3b7adfa8466e08b698b5577571133e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/