docs: note mockprock is consumed from source, not a released package - #1339

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5, actions/setup-python@e9aba2c848f5ebd159c070c61ea2c4e2b122355e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/