Restore an ability that class level `update` without giving ids

That ability was introduced at #11898 as `Relation#update` without
giving ids, so the ability on the class level is not documented and not
tested.

c83e30da which fixes #33470 has lost two undocumented abilities.

One has fixed at 5c656889, but I missed the ability on the class level.

Removing any feature should not be suddenly happened in a stable version
even if that is not documented.

I've restored the ability and added test case to avoid any regression in
the future.

Fixes #34743.

activerecord/lib/active_record/persistence.rb

@@ -96,11 +96,13 @@ def instantiate(attributes, column_types = {}, &block)
       # When running callbacks is not needed for each record update,
       # it is preferred to use {update_all}[rdoc-ref:Relation#update_all]
       # for updating all records in a single query.
-      def update(id, attributes)
+      def update(id = :all, attributes)
         if id.is_a?(Array)
           id.map { |one_id| find(one_id) }.each_with_index { |object, idx|
             object.update(attributes[idx])
           }
+        elsif id == :all
+          all.each { |record| record.update(attributes) }
         else
           if ActiveRecord::Base === id
             raise ArgumentError,

activerecord/test/cases/persistence_test.rb

@@ -53,6 +53,20 @@ def test_update_many_with_invalid_id
     assert_not_equal "2 updated", Topic.find(2).content
   end
 
+  def test_class_level_update_without_ids
+    topics = Topic.all
+    assert_equal 5, topics.length
+    topics.each do |topic|
+      assert_not_equal "updated", topic.content
+    end
+
+    updated = Topic.update(content: "updated")
+    assert_equal 5, updated.length
+    updated.each do |topic|
+      assert_equal "updated", topic.content
+    end
+  end
+
   def test_class_level_update_is_affected_by_scoping
     topic_data = { 1 => { "content" => "1 updated" }, 2 => { "content" => "2 updated" } }