Sorting: Renamed sort set to sort rule

Renamed based on feedback from Tim and Script on Discord.
Also fixed flaky test

database/factories/Entities/Models/BookFactory.php

@@ -27,7 +27,7 @@ class BookFactory extends Factory
             'slug'        => Str::random(10),
             'description' => $description,
             'description_html' => '<p>' . e($description) . '</p>',
-            'sort_set_id' => null,
+            'sort_rule_id' => null,
             'default_template_id' => null,
         ];
     }