Bump `google-protobuf` to v4.27

twirp.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.required_ruby_version = '>= 1.9'
-  spec.add_runtime_dependency 'google-protobuf', '~> 3.0', '>= 3.7.0'
+  spec.add_runtime_dependency 'google-protobuf', '>= 3.7.0', '< 5'
   spec.add_runtime_dependency 'faraday', '< 3' # for clients
   spec.add_dependency 'rack', '>= 2.2.3' # for service server handling