Fixes #35432 - Use Rails 6.1 defaults

Rails has introduced config.load_defaults[1] to load the defaults of a
certain version. Currently this is not called at all, which means using
Rails < 5.0 defaults. Changing this brings Foreman more in line with the
Rails recommended defaults.

[1]: https://guides.rubyonrails.org/configuring.html#versioned-default-values

config/application.rb

@@ -100,7 +100,7 @@ class Foreman::Consoletie < Rails::Railtie
 
 module Foreman
   class Application < Rails::Application
-    config.load_defaults 6.0
+    config.load_defaults 6.1
 
     # Rails 5.0 changed this to true, but a lot of code depends on this
     config.active_record.belongs_to_required_by_default = false