[ci skip] Active Storage: updating associations replaces

In Rails updating an Active Storage relation will now replace the entire
association instead of merely adding to it.
rails#35817 (comment)

Fixes rails#35817

cc @georgeclaghorn

guides/source/6_0_release_notes.md

@@ -453,6 +453,8 @@ Please refer to the [Changelog][active-storage] for detailed changes.
 
 ### Notable changes
 
+*   Updating an attached model via `update` or `update!` ala `@user.update!(images: [ … ])` now replaces the existing images instead of merely adding to them.
+
 Active Model
 ------------