Upgrade to Jekyll 4

[arthurattwell]

Jekyll 4's build times are much faster than v3, which is a major reason to switch. In early testing, the Electric Book template doesn't work with Jekyll 4 possibly because, it seems, some of our variables that define paths in _includes/metadata aren't giving correct values. For instance, some pages' links to JS and CSS files are building incorrectly. This may be a result of changes to the way paths are handled in Jekyll, or the way templates are now rendered.

So we need to do some thorough testing and make fixes, and support for Jekyll 4 will require a new release. The Jekyll upgrade guide should be useful.

[arthurattwell]

Further testing suggests that the latest EBT works fine with Jekyll 4.2 without any further changes. Either something changed in Jekyll or the EB template since Jekyll 4, or my early testing was faulty.

Note that GitHub Pages still only supports Jekyll 3.9. So our default will remain the github-pages gem, and by implication, for now, Jekyll 3.9. Users who don't need GitHub Pages can set their own Jekyll version in Gemfile.

[arthurattwell]

PS Our earlier issues might have been caused by a caching problem in Jekyll 4.0 which was subsequently fixed in jekyll/jekyll#7967.