README: separate the primary target from secondary support - #9

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: ilammy/msvc-dev-cmd@0b201ec74fa43914dc39ae48a89fd1d8cb592756. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/