Lock concurrent-ruby version

To fix
```
/home/runner/work/solidus_tracking/solidus_tracking/vendor/bundle/ruby/3.1.0/gems/activesupport-7.0.8.7/lib/active_support/logger_thread_safe_level.rb:12:in `<module:LoggerThreadSafeLevel>': uninitialized constant ActiveSupport::LoggerThreadSafeLevel::Logger (NameError)

    Logger::Severity.constants.each do |severity|
    ^^^^^^
```

This could be removed once we're solely testing on Rails 7.1+, but
because this gem is a bit outdated (Solidus < 4) we need it for now.

Author: kjriga

Gemfile

@@ -30,6 +30,8 @@ gem 'ostruct'
 # This is locked due to a 'stack level too deep' error/bug in v0.1.0.
 gem 'state_machines', '~> 0.6.0'
 
+gem 'concurrent-ruby', '1.3.4'
+
 case ENV['DB']
 when 'mysql'
   gem 'mysql2'