fix: update certifi and pin GitHub Actions (dev-26.3) - #1894

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: astral-sh/ruff-action@4919ec5cf1f49eff0871dbcea0da843445b837e6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/