Update dependent webrick version (#154)

This will solve CVE of GHSA-6f62-3596-g6w7 

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

Author: Watson1978

fluent-plugin-opensearch.gemspec

@@ -31,7 +31,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency "faraday_middleware-aws-sigv4", "~> 1.0.1"
 
   s.add_development_dependency 'rake', '>= 0'
-  s.add_development_dependency 'webrick', '~> 1.7.0'
+  s.add_development_dependency 'webrick', '~> 1.9'
   s.add_development_dependency 'webmock', '~> 3.18.1'
   s.add_development_dependency 'test-unit', '~> 3.3.0'
   s.add_development_dependency 'minitest', '~> 5.8'