Add ostruct as bundled gem

The ostruct gem promoted as a bundled from default gems from Ruby 3.5.0

More details:
 - https://bugs.ruby-lang.org/issues/20309
 - https://github.com/ruby/ruby/blob/cfca348436e0a9da2bb2d4402a4003601501ef0e/NEWS.md?plain=1#L24

Author: numbata

Gemfile

@@ -38,6 +38,9 @@ group :development, :test do
   unless ENV['MODEL_PARSER'] == 'grape-swagger-entity'
     gem 'grape-swagger-entity', git: 'https://github.com/ruby-grape/grape-swagger-entity'
   end
+
+  # Conditionally load 'ostruct' only if Ruby >= 3.5.0
+  gem 'ostruct' if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.5.0')
 end
 
 group :test do