ci document: fix method forwarding with recent Ruby

    ArgumentError: wrong number of arguments (given 2, expected 1) (ArgumentError)
    /home/runner/work/rexml/rexml/Rakefile:18:in `warn'
    /home/runner/work/rexml/rexml/Rakefile:18:in `warn'
    /opt/hostedtoolcache/Ruby/3.3.5/x64/bin/bundle:25:in `load'
    /opt/hostedtoolcache/Ruby/3.3.5/x64/bin/bundle:25:in `<main>'

Rakefile

@@ -14,7 +14,7 @@ task :default => :test
 namespace :warning do
   desc "Treat warning as error"
   task :error do
-    def Warning.warn(*message)
+    def Warning.warn(*message, **)
       super
       raise "Treat warning as error:\n" + message.join("\n")
     end