Add a deprecation warning to 8.x about the deletion of app/views/catalog/_constraints.html.erb

[sandbergja]

In our catalog, we had a custom view partial that had the line:

<%= render 'constraints' %>

This partial is removed in Blacklight 9. It would be nice to let blacklight implementers who have customized their view partials know that they should do something like this instead:

<%= render blacklight_config&.view_config(document_index_view_type)&.constraints_component.new(search_state: search_state) %>

[sandbergja]

Ah, I see this is part of #3330