gem: update google-protobuf to v4

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

.ruby-version

@@ -29,12 +29,12 @@ Gem::Specification.new do |spec|
   spec.test_files = test_files
   spec.require_paths = ['lib']
 
-  spec.add_development_dependency 'bundler', '~> 2.2'
+  spec.add_development_dependency 'bundler'
   spec.add_development_dependency 'rake', '~> 13.0'
   spec.add_development_dependency 'rubocop', '~> 1.22'
   spec.add_development_dependency 'rubocop-rake', '~> 0.6'
   spec.add_development_dependency 'test-unit', '~> 3.3'
   spec.add_runtime_dependency 'fluentd', ['>= 1.0', '< 2']
-  spec.add_runtime_dependency 'google-protobuf', ['~> 3.21']
+  spec.add_runtime_dependency 'google-protobuf', '~> 4.30'
   spec.add_runtime_dependency 'oj', '~> 3.13'
 end