Add CHANGELOG entry for #10879

[ci skip]

activesupport/CHANGELOG.md

+*   Add `Date#middle_of_day`, `DateTime#middle_of_day` and `Time#middle_of_day` methods.
+
+    Also added `midday`, `noon`, `at_midday`, `at_noon` and `at_middle_of_day` as aliases.
+
+    *Anatoli Makarevich*
+
 *   Fix ActiveSupport::Cache::FileStore#cleanup to no longer rely on missing each_key method.
 
     *Murray Steele*