searchable: replace find_ordered with in_order_of.

Rails 7 added an `in_order_of` method that does what our `find_ordered`
method did before.

app/models/post.rb

@@ -962,7 +962,7 @@ class Post < ApplicationRecord
         random_up(key) || random_down(key)
       end.compact.uniq
 
-      reorder(nil).find_ordered(posts.map(&:id))
+      reorder(nil).in_order_of(:id, posts.map(&:id))
     end
 
     def random_up(key)