refactor: reuse single discourse cache across API instances - #2831

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, dorny/paths-filter@d1c1ffe0248fe513906c8e24db8ea791d46f8590. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/