Ruby 3 Support

What's New:

• 🎈 Ruby 3 Support: We've added support for Ruby 3, ensuring you can use city-state with the latest version of Ruby.
• 🧪 Unit Tests with RSpec: To ensure the reliability of our gem, we've introduced unit tests using RSpec.
• 📦 Development Dependencies: Added rspec 3.10 to our development dependencies.

Improvements:

• 📚 MaxMind Database Update: We've updated the bundled MaxMind database, ensuring you have the latest city and state data.
• 🔧 Runtime Dependencies Upgrade: The minimum supported versions are now Rake 11.0 and Rubyzip 2.3.

Removals:

• ❌ Ruby 2.5 Support Dropped: To keep up with the latest advancements, we've dropped support for Ruby 2.5. The minimum supported Ruby version is now 2.6.0.
• 🧹 Cleaned Version Control: We've removed the Gemfile.lock from version control, aligning with best practices for gem development.