add security notes for stellar-core v28.0.0 release - #5442

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: namespacelabs/nscloud-checkout-action@v7. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/