Update dependencies and test with Ruby 4.0

https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/

We need to add `ostruct` as a dev dependency because it doesn't come
as a default gem with Ruby 4.0 anymore.

Lock minitest < 6 since v6 has some incompatibilities with released
versions of Rails that will be fixed in future versions.

This is something I didn't run into a few other gems, but SimpleForm
did, presumabily because it touches additional areas like AV tests.

Gemfile

@@ -14,12 +14,14 @@ gem "rails-controller-testing"
 gem "responders", "~> 3.1"
 
 group :test do
+  gem "minitest", "< 6"
   gem "mocha", "~> 2.1", require: false
   gem "omniauth-facebook"
   gem "omniauth-openid"
   gem "rexml"
   gem "timecop"
   gem "webrat"
+  gem "ostruct"
 end
 
 platforms :ruby do