entt_traits: relax class constraints for backward compatibility

src/entt/entity/entity.hpp

@@ -25,11 +25,7 @@ struct entt_traits<Type>: entt_traits<std::underlying_type_t<Type>> {
 };
 
 template<typename Type>
-requires requires {
-    typename Type::entity_type;
-    requires std::convertible_to<Type, typename Type::entity_type>;
-    requires std::constructible_from<Type, typename Type::entity_type>;
-}
+requires requires { typename Type::entity_type; }
 struct entt_traits<Type>
     : entt_traits<typename Type::entity_type> {
     using value_type = Type;