doc: fix wording in worker_threads.md - #64913

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: nodejs/node-pr-labeler@d4cf1b8b9f23189c37917000e5e17e796c770a6b. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/