Fix `rake yard` does not work with Ruby >= 3.2 due to nested array issue (#39576).

Patch by Go MAEDA.


git-svn-id: https://svn.redmine.org/redmine/trunk@22456 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/tasks/yardoc.rake

@@ -3,7 +3,7 @@ begin
 
   YARD::Rake::YardocTask.new do |t|
     files = ['app/**/*.rb']
-    files << Dir['lib/**/*.rb', 'plugins/**/*.rb'].reject {|f| f.match(/test/) }
+    files += Dir['lib/**/*.rb', 'plugins/**/*.rb'].reject {|f| f.match(/test/)}
     t.files = files
 
     static_files = ['doc/CHANGELOG',