Don't cast `stale_state` to String

I'm looking at reducing the allocations and memory footprint
of Active Record models, and based on a small benchmark I saw
a lot of numeric strings being created from `stale_state`.

I tracked this cast all the way down to 1c07b84df9
but unfortunately the commit message doesn't explain why it was added.

I can't think of a reason why this would be needed.

The benchmark is essentially just: `Post.includes(:comments).to_a` with
`100` posts and `20` comments each.

```
Total allocated: 4.69 MB (45077 objects)
Total retained: 3.84 MB (29623 objects)

retained objects by location
-----------------------------------
...
      2000  activerecord/lib/active_record/associations/belongs_to_association.rb:152

retained memory by location
-----------------------------------
...
  80.00 kB  activerecord/lib/active_record/associations/belongs_to_association.rb:152
```

NB: This break the final assertion of the Rails 6.1 Marshal backward compatibility
test, but I think it's an acceptable tradeoff.

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -148,8 +148,7 @@ module ActiveRecord
         end
 
         def stale_state
-          result = owner._read_attribute(reflection.foreign_key) { |n| owner.send(:missing_attribute, n, caller) }
-          result && result.to_s
+          owner._read_attribute(reflection.foreign_key) { |n| owner.send(:missing_attribute, n, caller) }
         end
     end
   end

activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb

@@ -41,8 +41,9 @@ module ActiveRecord
         end
 
         def stale_state
-          foreign_key = super
-          foreign_key && [foreign_key.to_s, owner[reflection.foreign_type].to_s]
+          if foreign_key = super
+            [foreign_key, owner[reflection.foreign_type]]
+          end
         end
     end
   end

activerecord/lib/active_record/associations/through_association.rb

@@ -82,7 +82,7 @@ module ActiveRecord
         def stale_state
           if through_reflection.belongs_to?
             Array(through_reflection.foreign_key).filter_map do |foreign_key_column|
-              owner[foreign_key_column] && owner[foreign_key_column].to_s
+              owner[foreign_key_column]
             end.presence
           end
         end

activerecord/test/cases/marshal_serialization_test.rb

@@ -33,7 +33,6 @@ class MarshalSerializationTest < ActiveRecord::TestCase
     assert_equal "Have a nice day", topic.content
     assert_predicate topic.association(:replies), :loaded?
     assert_predicate topic.replies.first.association(:topic), :loaded?
-    assert_same topic, topic.replies.first.topic
   end
 
   def test_deserializing_rails_7_1_marshal_basic