Update required ruby version of this gem

Author: satoryu

rakuten_web_service.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files`.split($/)
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ['lib']
-  spec.required_ruby_version = '>= 2.7.0'
+  spec.required_ruby_version = '>= 3.1.0'
 
   spec.add_dependency 'base64', '~> 0.2'
   spec.add_dependency 'bigdecimal', '~> 3.1'