I'm trying to upgrade from version 12.5 to the version 13.1 in RDS. But I'm receiving an error similar to the one described in this question: RDS does not support creating a DB instance with the following combination. As far as I understand in order to upgrade a proper version DB instance class should be selected. However the error message shows a previous selection, not a new one. Same message is received in Terraform setting up
engine_version = "13.1"
instance_class = "db.t3.small"
Screenshot from AWS showing the changes and message:

Copied error message to make it easier to research:
We're sorry, your request to modify DB instance [service-name] has failed. RDS does not support creating a DB instance with the following combination: DBInstanceClass=db.t2.small, Engine=postgres, EngineVersion=13.1, LicenseModel=postgresql-license. For supported combinations of instance class and database engine version, see the documentation.
Is there some key to allow_instance_class_change (a made up key) or something else I overlooked what prevents an upgrade? Thank you!