Add scheduled Lychee link check - #206

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/labeler@ac9175f8a1f3625fd0d4fb234536d26811351594. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/