fix: update certifi and pin GitHub Actions (dev-25.12) - #1895

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: kentaro-m/auto-assign-action@f4648c0a9fdb753479e9e75fc251f507ce17bb7e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/