Add changelog entry for 3110caec [ci skip]

Since it is changes of the public API, it seems valuable to add a mention
about it to the changelog file.

Follow up 3110caec

activejob/CHANGELOG.md

+*   Allow passing multiple exceptions to `retry_on`, and `discard_on`.
+
+    *George Claghorn*
+
 *   Pass the error instance as the second parameter of block executed by `discard_on`.
 
     Fixes #32853.