docs: document subscription charge payer/spender binding - #1771

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea, step-security/harden-runner@002fdce3c6a235733a90a27c80493a3241e56863. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/