Keep MultiIndex columns in GroupBy.agg for MultiIndex-column sources - #23366

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions-ecosystem/action-add-assignees@ce5019e63cc4f35aba27308dc88d19c8f3686747. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/