More Node.js 20 deprecation warnings

[troglobit]

The User Guide builder was missed in #1437:

Warning

build
Node.js 20 actions are deprecated. The following actions are running on Node.js 20
and may not work as expected: actions/checkout@v4, actions/upload-artifact@v4.
Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026.
Node.js 20 will be removed from the runner on September 16th, 2026. Please check if
updated versions of these actions are available that support Node.js 24. To opt into
Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable
on the runner or in your workflow file. Once Node.js 24 becomes the default, you can
temporarily opt out by setting ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true.

For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

[troglobit]

I suggest we fix this in the next release cycle, v26.06