[ruby/rdoc] Dir.children is 2.5+

RDoc seems still supporting outdated 2.4.

https://github.com/ruby/rdoc/runs/2565344070?check_suite_focus=true#step:5:64
```
Error: test_remove_unparseable_CVE_2021_31799(TestRDocRDoc): NoMethodError: undefined method `children' for Dir:Class
/home/runner/work/rdoc/rdoc/test/rdoc/test_rdoc_rdoc.rb:467:in `block in test_remove_unparseable_CVE_2021_31799'
     464:       end
     465:
     466:       assert_equal file_list, @rdoc.remove_unparseable(file_list)
  => 467:       assert_equal file_list, Dir.children('.')
     468:     end
     469:   end
     470:
```

https://github.com/ruby/rdoc/commit/5a4a64dc0f

test/rdoc/test_rdoc_rdoc.rb

@@ -464,7 +464,7 @@ class TestRDocRDoc < RDoc::TestCase
       end
 
       assert_equal file_list, @rdoc.remove_unparseable(file_list)
-      assert_equal file_list, Dir.children('.')
+      assert_equal file_list, Dir.entries('.') - %w[. ..]
     end
   end