fix: Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-ACTIVERECORD-11800112

Gemfile

@@ -8,13 +8,13 @@ group :development, :test do
   gem "rake"
   gem "pry"
   gem "pry-rails"
-  gem "test-unit-rails"
+  gem "test-unit-rails", ">= 6.0.0"
   gem "test-unit-notify"
 end
 
 group :development do
   gem 'listen', '>= 3.0.5', '< 3.2'
-  gem 'i18n_generators', '2.1.1'
+  gem 'i18n_generators', '2.2.0'
   gem 'better_errors'
   gem 'web-console', '~> 3.6'
   gem 'binding_of_caller'