Improve test support for newer Rails versions

[adfoster-r7]

It looks like the rails dummy application doesn't call config.load_defaults '7.1' https://api.rubyonrails.org/v8.0.2/classes/Rails/Application/Configuration.html#method-i-load_defaults -

i.e. for our more generic scenario config.load_defaults [ActionView::VERSION::MAJOR, ActionView::VERSION::MINOR].join('.') etc:

https://github.com/rapid7/metasploit_data_models/blob/ac923620ef8df85c0b7b456dd907323b792fed65/spec/dummy/config/application.rb