Update Redcarpet to 3.6 (#38220).

git-svn-id: https://svn.redmine.org/redmine/trunk@22082 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -46,7 +46,7 @@ end
 
 # Optional Markdown support
 group :markdown do
-  gem 'redcarpet', '~> 3.5.1'
+  gem 'redcarpet', '~> 3.6.0'
 end
 
 # Optional CommonMark support, not for JRuby