docs(rails): explain disabling agent, fixes #1466 - #1652

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: AlexanderWert/issue-labeler@v2.3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/