Fix callback to only run before update

`before_save` doesn't accept `:on` as an argument, so this callback was actually running on all save events, not just on update.

rails/rails#17622

Depending on what `reset_timestamp_attrs_for_update_if_needed` does, this might need to be backported to older versions.

I've submitted a PR to Rails to raise an exception when unexpected arguments are passed... unsure if an exception is the right way to go when there might be quite a few gems with this bug.

rails/rails#30919

lib/paper_trail/model_config.rb

@@ -59,7 +59,7 @@ def on_destroy(recording_order = "before")
 
     # Adds a callback that records a version after an "update" event.
     def on_update
-      @model_class.before_save(on: :update) { |r|
+      @model_class.before_update { |r|
         r.paper_trail.reset_timestamp_attrs_for_update_if_needed
       }
       @model_class.after_update { |r|