Use PetState#updated_at for the supported pose cache key, not latest ID

This is because labeling poses with the Support tools *should*
invalidate the `PetState.all_supported_poses` cache! But the previous
cache key would only invalidate when a new pet state is *added*, not
when one is *edited*.

app/models/pet_state.rb

@@ -211,7 +211,7 @@ class PetState < ApplicationRecord
   end
 
   def self.last_updated_key
-    PetState.maximum(:id)
+    PetState.maximum(:updated_at)
   end
 
   def self.all_supported_poses