Change title of backlog list to Open

Right now this is just overriden in the frontend and led to bugs in
other components where we forgot to override the title.

This moves the logic to the backend so that we don't have to do those
overrides.

app/models/concerns/boards/listable.rb

@@ -34,7 +34,13 @@ module Boards
     end
 
     def title
-      label? ? label.name : list_type.humanize
+      if label?
+        label.name
+      elsif backlog?
+        _('Open')
+      else
+        list_type.humanize
+      end
     end
 
     private

changelogs/unreleased/rename-backlog-list-to-open-in-backend.yml

+---
+title: Rename Backlog list to Open in issue boards
+merge_request: 51562
+author:
+type: fixed

spec/support/shared_examples/models/boards/listable_shared_examples.rb

@@ -68,6 +68,12 @@ RSpec.shared_examples 'boards listable model' do |list_factory|
       expect(subject.title).to eq 'Development'
     end
 
+    it 'returns Open when list_type is set to backlog' do
+      subject.list_type = :backlog
+
+      expect(subject.title).to eq 'Open'
+    end
+
     it 'returns Closed when list_type is set to closed' do
       subject.list_type = :closed