Fix generate-mindmap GHA workflow #570

When running our generate-mindmap GitHub Action workflow, we get these two warnings:

The following actions uses node12 which is deprecated and will be forced to run on node16: swinton/screenshot-website@v1.x. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/Show

The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Currently the screenshot of our mindmap still gets generated correctly by the generate-mindmap workflow. However at some point in the not too distant future this will stop working.

Looks like both warnings are related to the swinton/screenshot-website GHA that we are using for generating a screenshot of the HTML generated by markmap. However that GHA does not seem to be maintained anymore, although we are trying to reach the maintainer here.

What options do we have?

1. switch to another GHA that can create a screenshot from HTML
2. help fix swinton/screenshot-website and create a new release that we (and others) can use

Option (1) might be the quickest, although (2) would also be neat in the spirit of open source :)