Add threat model for the policy repo - #1794

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: conforma/pr-size-label-action@v1.0.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/