Use the native Class#descendants if available

Ruby 3.1 is very likely to ship with a native implementation
of `Class#descendants` that doesn't need to iterate over
ObjectSpace. So we should use it if available.

Ref: https://bugs.ruby-lang.org/issues/14394
Ref: https://github.com/ruby/ruby/pull/4974

activesupport/lib/active_support/core_ext/class/subclasses.rb

@@ -18,7 +18,7 @@ class Class
     ObjectSpace.each_object(singleton_class).reject do |k|
       k.singleton_class? || k == self
     end
-  end
+  end unless method_defined?(:descendants) # RUBY_VERSION >= "3.1"
 
   # Returns an array with the direct children of +self+.
   #