Update the marshalized object fixtures' version from 6.0 to 6.1

It is a blocker to remove no-op constants in the main branch.

activerecord/test/cases/marshal_serialization_test.rb

@@ -7,17 +7,17 @@
 class MarshalSerializationTest < ActiveRecord::TestCase
   fixtures :topics
 
-  def test_deserializing_rails_6_0_marshal_basic
-    topic = Marshal.load(marshal_fixture("rails_6_0_topic"))
+  def test_deserializing_rails_6_1_marshal_basic
+    topic = Marshal.load(marshal_fixture("rails_6_1_topic"))
 
     assert_not_predicate topic, :new_record?
     assert_equal 1, topic.id
     assert_equal "The First Topic", topic.title
     assert_equal "Have a nice day", topic.content
   end
 
-  def test_deserializing_rails_6_0_marshal_with_loaded_association_cache
-    topic = Marshal.load(marshal_fixture("rails_6_0_topic_associations"))
+  def test_deserializing_rails_6_1_marshal_with_loaded_association_cache
+    topic = Marshal.load(marshal_fixture("rails_6_1_topic_associations"))
 
     assert_not_predicate topic, :new_record?
     assert_equal 1, topic.id