Updates the graphql-ruby minimum supported version to 1.13.0

[Samsinite]

As requested, this commit updates the graphql-ruby testing matrix to include ~1.13.0, ~2.0.0, ~2.1.0 and ~2.2.0. I excluded ~1.12.0 due to failing the CI tests when it was included in the matrix, and specified 1.13.0 as the minimum supported version in the gemspec.

For reference, the ~1.12.0 failure was:

/opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/graphql-1.12.24/lib/graphql/schema/argument.rb:55:in `initialize': missing keyword: :required (ArgumentError)
    from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/graphql-1.12.24/lib/graphql/schema/member/accepts_definition.rb:142:in `initialize'
    from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/graphql-1.12.24/lib/graphql/schema/member/has_arguments.rb:38:in `new'
    from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/graphql-1.12.24/lib/graphql/schema/member/has_arguments.rb:38:in `argument'
    from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/graphql-1.12.24/lib/graphql/schema/input_object.rb:124:in `argument'
    from /home/runner/work/graphql-client/graphql-client/test/test_schema.rb:20:in `<class:NodeArgInput>'
    from /home/runner/work/graphql-client/graphql-client/test/test_schema.rb:18:in `<class:TestSchemaType>'
    from /home/runner/work/graphql-client/graphql-client/test/test_schema.rb:7:in `<top (required)>'

[rmosolgo]

Awesome, thank you!