Replace OpenStruct

[kuredev]

This library uses OpenStruct to generate API responses.
https://github.com/microcmsio/microcms-ruby-sdk/blob/v1.2.0/lib/microcms.rb#L33

But OpenStruct is officially discouraged since Ruby 3.0.0.
https://docs.ruby-lang.org/ja/latest/doc/news=2f3_0_0.html

I think, to upgrade Ruby 3.0 or later, OpenStruct should be replated.