Address to "DEPRECATION WARNING: Uniqueness validator will no longer enforce case sensitive comparison in Rails 6.1"

Caused by #36210.

activerecord/test/schema/schema.rb

@@ -115,7 +115,7 @@ ActiveRecord::Schema.define do
     t.column :font_size, :integer, **default_zero
     t.column :difficulty, :integer, **default_zero
     t.column :cover, :string, default: "hard"
-    t.string :isbn
+    t.string :isbn, **case_sensitive_options
     t.datetime :published_on
     t.index [:author_id, :name], unique: true
     t.index :isbn, where: "published_on IS NOT NULL", unique: true