Fix appraisal and bundler version CI conflicts

[bensheldon]

Fixes problem by pinning thoughtbot/appraisal#174

Also took opportunity to simplify the Github Actions:

• Fully use ruby/setup-ruby action's bundler version and gem caching
• Simplify the ruby/pg matrix
• libpg-dev has been added to the base image: Add libpq-dev actions/runner-images#12 and the line about "Install missing libs" can be removed.

Response to seeing errors like:

•  Run bundle exec appraisal install --without lint --path $BUNDLE_PATH
   bundler: failed to load command: appraisal (/home/runner/work/good_job/good_job/vendor/bundle/ruby/2.7.0/bin/appraisal)
   /opt/hostedtoolcache/Ruby/2.7.2/x64/lib/ruby/site_ruby/2.7.0/bundler/runtime.rb:302:in `check_for_activated_spec!': You have already activated bundler 2.2.11, but your Gemfile requires bundler 2.2.13. Since bundler is a default gem, you can either remove your dependency on it or try updating to a newer version of bundler that supports bundler as a default gem. (Gem::LoadError)
  

• Trying to register Bundler::GemfileError for status code 4 but Bundler::GemfileError is already registered