Upgrade Rails gem to latest v6.x

[murny]

• Upgrade gem to Rails 6
• Can run the update task: https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#the-update-task
  • Can also check out http://railsdiff.org/ to see diffs of a rails new app throughout version changes
• Run rails with the old defaults for now (in later issues we will tackle the bigger upgrades such as webpacker or zeitwerk)
• Follow through official upgrade docs: https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-5-2-to-rails-6-0 and fix any changes/deprecations
• Enable parallel testing (EDIT: Cannot complete until Post Fedora Automated Test Cleanup #1445 is taken on first)
• Maybe convert community/collection description textareas to use ActionText as a proof of concept? As these are admin only and be a good candidate 🤔

Should have a working build of code on Rails 6 after the above is done (using sprockets instead of webpacker and no zeitwerk/etc)

[murny]

PR has been merged and work completed (parallel testing work pushed off to another issue #1445. ActionText was a nice to have, but going to push this off until we have a real reason to use it)