Tests fail with Ruby 3.3.1

[ajtucker]

Setting .ruby-version to 3.3.1, running make setup and make test results in a test failure:

sapi-client-ruby/lib/sapi_client/hierarchy.rb:5:in `<module:SapiClient>': uninitialized constant SapiClient::OpenStruct (NameError)

  SKOS_SCHEME = OpenStruct.new(
                ^^^^^^^^^^
        from /home/alex/Development/Epimorphics/sapi-client-ruby/lib/sapi_client/hierarchy.rb:3:in `<top (required)>'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from /home/alex/Development/Epimorphics/sapi-client-ruby/lib/sapi_client.rb:20:in `<top (required)>'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from /home/alex/Development/Epimorphics/sapi-client-ruby/test/test_helper.rb:17:in `<top (required)>'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from /home/alex/Development/Epimorphics/sapi-client-ruby/test/sapi_client/application_test.rb:3:in `<top (required)>'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from <internal:/home/alex/.rbenv/versions/3.3.1/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
        from /home/alex/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:21:in `block in <main>'
        from /home/alex/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:6:in `select'
        from /home/alex/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:6:in `<main>'
rake aborted!
Command failed with status (1)

Note that OpenStruct is "officially discouraged" and there are some Ruby3 gotchas about lazy evaluation.

We should add the max Ruby version this works with to spec.required_ruby_version.