Upgrade the Ruby Version 3.0 or Later

This library supports Ruby 2.7.2.
https://github.com/microcmsio/microcms-ruby-sdk/blob/v1.2.0/.ruby-version

But Ruby 2.7 has been EoLed at the end of March 2023.
https://endoflife.date/ruby

I think this library should upgrade the Ruby Version to 3.x or later.🤔 
