docs: name the five supported targets and drop Plan 9 as a review gate - #986

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, actions/github-script@f28e40c7f34bde8b3046d885e986cb6290c5673b, actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/