Increment 21c: measurements for choosing 21d (C, A1, A0, evaluate-once); 21d deferred - #104

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