Gemfile: add a gem which used to be standard library; use unreleased Jekyll which works

[olleolleolle]

This PR makes bundle exec jekyll serve work

• use the not-yet-released version of Jekyll - see its https://github.com/jekyll/jekyll/blob/master/History.markdown for more details ("Add support for upcoming logger 1.4.3 (#9392)")
• supply a needed gem which was standard library...but is now demoted to not-always-included.

This stops warnings like

[...]/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/safe_yaml-1.0.5/lib/safe_yaml/transform.rb:1: warning: base64 was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec. Also contact author of safe_yaml-1.0.5 to add base64 into its gemspec.

From happening at start of the app.