Deprecation warning: custom-task-type kata

[michaelin]

When running the custom-task-types exercise with --warning-mode all (gradle version 6.2):

Property 'message' is not annotated with an input or output annotation. This behaviour has been deprecated and is scheduled to be removed in Gradle 7.0. See https://docs.gradle.org/6.2/userguide/more_about_tasks.html#sec:up_to_date_checks for more details.