Register new autoloading in AS CHANGELOG [ci skip]

There is too much to say about it for a CHANGELOG entry, and linking to
the original PR could mislead if there are later changes as already
happened with the gem dependency, so just a one-liner.

For final we'll up to date documentation.

activesupport/CHANGELOG.md

+*   New autoloading based on [Zeitwerk](https://github.com/fxn/zeitwerk).
+
+    *Xavier Noria*
+
 *   Revise `ActiveSupport::Notifications.unsubscribe` to correctly handle Regex or other multiple-pattern subscribers.
 
     *Zach Kemp*