fix runtime dependency

since #67, doorkeeper has called an API (`#mapping`) that was introduced in 5.0. See more here: 3893ea1#r32599682

doorkeeper-openid_connect.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = ">= 2.3"
 
-  spec.add_runtime_dependency 'doorkeeper', '>= 4.3'
+  spec.add_runtime_dependency 'doorkeeper', '>= 5.0'
   spec.add_runtime_dependency 'json-jwt', '~> 1.6'
 
   spec.add_development_dependency 'rspec-rails'