Updates the requirements on [jekyll](https://github.com/jekyll/jekyll) to permit the latest version.
- [Release notes](https://github.com/jekyll/jekyll/releases)
- [Changelog](https://github.com/jekyll/jekyll/blob/master/History.markdown)
- [Commits](https://github.com/jekyll/jekyll/compare/v3.8.5...v4.0.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>