Drop support for Rails 5.x (#2201)

Rails 5.0 and 5.1 already reached end-of-life, and 5.2 will reach it
next week. Time to drop support for them.

Additionally, removed gemfiles/sass_3_4.gemfile, which was a
leftover from #1197

https://guides.rubyonrails.org/maintenance_policy.html#severe-security-issues
#1197

.circleci/config.yml

@@ -55,9 +55,6 @@ jobs:
       - shared_steps
       # Run the tests against the versions of Rails that support Ruby 2.7
       - run: bundle exec appraisal install
-      - run: bundle exec appraisal rails50 rspec
-      - run: bundle exec appraisal rails51 rspec
-      - run: bundle exec appraisal rails52 rspec
       - run: bundle exec appraisal rails60 rspec
       - run: bundle exec appraisal rails61 rspec
       - run: bundle exec appraisal rails70 rspec

Appraisals

@@ -1,35 +1,3 @@
-appraise "rails50" do
-  gem "actionpack", "~> 5.0.0"
-  gem "actionview", "~> 5.0.0"
-  gem "activerecord", "~> 5.0.0"
-  gem "pg", "0.21.0"
-
-  group :development, :test do
-    gem "rspec-rails"
-  end
-end
-
-appraise "rails51" do
-  gem "actionpack", "~> 5.1.0"
-  gem "actionview", "~> 5.1.0"
-  gem "activerecord", "~> 5.1.0"
-  gem "pg", "0.21.0"
-
-  group :development, :test do
-    gem "rspec-rails"
-  end
-end
-
-appraise "rails52" do
-  gem "actionpack", "~> 5.2.0"
-  gem "actionview", "~> 5.2.0"
-  gem "activerecord", "~> 5.2.0"
-
-  group :development, :test do
-    gem "rspec-rails"
-  end
-end
-
 appraise "rails60" do
   gem "rails", "~> 6.0.3.4"
 end

app/helpers/administrate/application_helper.rb

@@ -4,11 +4,7 @@ module ApplicationHelper
     SINGULAR_COUNT = 1
 
     def application_title
-      if Rails::VERSION::MAJOR <= 5
-        Rails.application.class.parent_name.titlecase
-      else
-        Rails.application.class.module_parent_name.titlecase
-      end
+      Rails.application.class.module_parent_name.titlecase
     end
 
     def render_field(field, locals = {})

spec/example_app/config/application.rb

@@ -25,15 +25,7 @@ class Application < Rails::Application
     end
 
     config.action_controller.action_on_unpermitted_parameters = :raise
-
-    if Rails::VERSION::MAJOR < 5
-      # Do not swallow errors in after_commit/after_rollback callbacks.
-      config.active_record.raise_in_transactional_callbacks = true
-    end
-
-    if Rails::VERSION::MAJOR >= 5
-      config.active_record.time_zone_aware_types = %i(datetime time)
-    end
+    config.active_record.time_zone_aware_types = %i(datetime time)
 
     # Opt-out of FLoC: https://amifloced.org/
     config.action_dispatch.