L-shaped: fix subproblem indexing when root_scenarios is used - #817

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3, actions/upload-artifact@v4, conda-incubator/setup-miniconda@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

WARNING conda.conda_pypi.main:notify_externally_managed_future(156): Did you know? You can install many PyPI packages with conda using the conda-pypi beta. Get started: https://docs.conda.io/projects/conda/en/stable/new-features.html