Bump quantecon-book-theme from 0.20.0 to 0.21.0 - #81

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, conda-incubator/setup-miniconda@v3, dawidd6/action-download-artifact@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

`auto-activate-base` is deprecated. Please use `auto-activate`. If your installer does not use the `base` environment as the default environment, also add `activate-environment: base`.