Load Rails 6.1 default configuration

After upgrading to Rails 6.1, we should load 6.1 defaults by calling load_defaults '6.1' so that we're closer to Rails defaults. We can explicitly change some of these settings where we don't want to follow defaults.

https://guides.rubyonrails.org/configuring.html#results-of-config-load-defaults has a list of configuration values.

This would also avoid us from explicitly setting some values just to follow the new default like in #332246 (closed)