elevate: Document ISO-based air-gapped upgrades in the NG testing guide - #839

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020, autofix-ci/action@635ffb0c9798bd160680f18fd73371e355b85f27. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/