Clarify (draft) PRs in contributing guidelines. - #541

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: docker/login-action@v3, warjiang/setup-skopeo@v0.1.3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/