Disable legacy_connection_handling on Rails 6.1+ (#308)

A new Active Record connection handler was added in Rails 6.1, however,
the old one remains the default for upgraded applications and throws
deprecation warnings under Rails 7

spec/dummy/config/application.rb

@@ -15,5 +15,9 @@ class Application < Rails::Application
     if Rails.gem_version < Gem::Version.new("6.0") && config.active_record.sqlite3
       config.active_record.sqlite3.represent_boolean_as_integer = true
     end
+
+    if Rails.gem_version >= Gem::Version.new("6.1")
+      config.active_record.legacy_connection_handling = false
+    end
   end
 end