fix: avoid SymPy 1.13 simplification slowdown - #596

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-python@v5, codecov/codecov-action@v3, hidakatsuya/action-setup-diff-pdf@v1.4.0, wtfjoke/setup-tectonic@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/