Update jekyll deps

[gracegrimwood]

General cleanup tasks for Jekyll dependencies and plugins:

• Removes unused dependencies from Gemfile (including jekyll-assets, which appears to be abandoned)
• Fixes some inconsistent formatting in the Gemfile
• Swaps out github-pages gem for jekyll, as the former is meant for legacy GitHub Pages deployments (which we aren't using) while the latter is more up to date. Also updates README.md to reflect this change
• Adds the jekyll-sass-converter plugin to the list of plugins in _config.yml, so it picks up and processes the Bootstrap sources in the _sass/ directory
• Updates all remaining dependencies and bumps the Bundler version used to install them

[gracegrimwood]

Forgot to mention: these changes have been tested and are live at https://gracegrimwood.github.io/kroxylicious.github.io/. Just be aware that the docs link doesn't work because we don't ship the docs in this repo.

[gracegrimwood]

I don't know of a way to confirm that the security issues are all addressed.

@k-wall FYI you can test this locally by installing the bundler-audit gem and running bundle audit. It will generate a list of vulnerabilities from the project's Gemfile. 🙂