doc: clarify minimum C++ standard - #1754

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: step-security/harden-runner@58077d3c7e43986b6b15fba718e8ea69e387dfcc. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/