Update to node20

[danielhjacobs]

Node 16 is deprecated. See https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

[danielhjacobs]

Well, seeing this get no attention, hopefully most users of this action can have their use cases covered with https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter. For reference, for Ruffle that was done in this PR: ruffle-rs/ruffle#16044