chore: update CI actions to current majors (checkout, cache, pnpm, volta) - #214

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: pnpm/action-setup@v4, volta-cli/action@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/