Switch out comments_disabled for diff_notes_disabled.

app/views/discussions/_jump_to_next.html.haml

--if @merge_request_diff
-  - comments_disabled = @merge_request_diff.latest? && !!@start_sha
+- diff_notes_disabled = (@merge_request_diff.latest? && !!@start_sha) if @merge_request_diff
 - discussion = local_assigns.fetch(:discussion, nil)
 - if current_user
   %jump-to-discussion{ "inline-template" => true, ":discussion-id" => "'#{discussion.try(:id)}'" }
@@ -8,5 +7,5 @@
         title: "Jump to next unresolved discussion",
         "aria-label" => "Jump to next unresolved discussion",
         data: { container: "body" },
-        disabled: comments_disabled }
+        disabled: diff_notes_disabled }
         = custom_icon("next_discussion")