Update Jekyll and packages

[bjohansebas]

As I mentioned #1736 (comment), Jekyll is outdated and many of its dependencies are as well. We will likely need to remove pages-gem since it is not actively maintained by GitHub

Which means we will probably now have to upload the page via GitHub Actions.

[bjohansebas]

related https://jekyllrb.com/docs/continuous-integration/github-actions/

[bjohansebas]

Now we have full control over the production environment, we can update our dependencies and use any plugin we might need.