security: remove or force-upgrade `rack`?

[SethTisue]

I submitted #1148 to upgrade rack, but it didn't actually get us to a new enough rack version to make the security warning go away

I tried adding gem 'rack', '>= 2.1.4' to Gemfile and then running bundle update --all but the result was:

Bundler could not find compatible versions for gem "rack":
  In Gemfile:
    rack (>= 2.1.4)

    rack-jekyll was resolved to 0.5.0, which depends on
      rack (~> 1.5)

but 0.5.0 is the latest available version of rack-jekyll, as per https://github.com/adaoraul/rack-jekyll and https://rubygems.org/gems/rack-jekyll/versions/0.5.0

[SethTisue]

possible paths forward here:

• figure out whether we actually need rack-jekyll for anything and if we don't, remove it
• force rack onto a newer version than rack-jekyll thinks it can use, and see if anything breaks
• ???

opinions? volunteer?

[SethTisue]

rack-jekyll came in in 2017 as part of #679 and seems to be a Heroku thing. we aren't using Heroku, so my best guess is that somebody at 47 Degrees was using Heroku during development, perhaps to deploy a staging version of the site

so we presumably don't need it anymore. I've submitted #1158 to remove it