Add Ruby 3.0 support (#16046)

* Fix issues with POSIX::Spawn, Terrapin and Ruby 3.0

Also improve the Terrapin monkey-patch for the stderr/stdout issue.

* Fix keyword argument handling throughout the codebase

* Monkey-patch Paperclip to fix keyword arguments handling in validators

* Change validation_extensions to please CodeClimate

* Bump microformats from 4.2.1 to 4.3.1

* Allow Ruby 3.0

* Add Ruby 3.0 test target to CircleCI

* Add test for admin dashboard warnings

* Fix admin dashboard warnings on Ruby 3.0

spec/controllers/admin/dashboard_controller_spec.rb

@@ -3,9 +3,19 @@
 require 'rails_helper'
 
 describe Admin::DashboardController, type: :controller do
+  render_views
+
   describe 'GET #index' do
-    it 'returns 200' do
+    before do
+      allow(Admin::SystemCheck).to receive(:perform).and_return([
+        Admin::SystemCheck::Message.new(:database_schema_check),
+        Admin::SystemCheck::Message.new(:rules_check, nil, admin_rules_path),
+        Admin::SystemCheck::Message.new(:sidekiq_process_check, 'foo, bar'),
+      ])
       sign_in Fabricate(:user, admin: true)
+    end
+
+    it 'returns 200' do
       get :index
 
       expect(response).to have_http_status(200)