Migrate to Rails 6.1 connection handling

This is required so that we can start using the Rails 6.1 connected_to
APIs. e.g. Ci.connected_to(role: :reading)

config/application.rb

@@ -165,6 +165,10 @@ module Gitlab
     # like if you have constraints or database-specific column types
     config.active_record.schema_format = :sql
 
+    # Use new connection handling so that we can use Rails 6.1+ multiple
+    # database support.
+    config.active_record.legacy_connection_handling = false
+
     config.action_mailer.delivery_job = "ActionMailer::MailDeliveryJob"
 
     # Enable the asset pipeline

config/environments/production.rb

@@ -54,6 +54,9 @@ Rails.application.configure do
   # Enable serving of images, stylesheets, and JavaScripts from an asset server
   config.action_controller.asset_host = ENV['GITLAB_CDN_HOST'] if ENV['GITLAB_CDN_HOST'].present?
 
+  # We use a env var to keep at old default until we enable this for GitLab.com
+  config.active_record.legacy_connection_handling = !Gitlab::Utils.to_boolean(ENV.fetch('ENABLE_RAILS_61_CONNECTION_HANDLING', false))
+
   # Do not dump schema after migrations.
   config.active_record.dump_schema_after_migration = false