Ignoring changes in name means that one can never run terraform deployments beyond the initial creation of the webapp service. Terraform will always modify a service in-place but try to use the same name which negates the point of setting the name in the google_cloud_run_service.webapp resource.
The downside of this is that one can't then deploy changes without deleting and re-creating random_id.webapp_revision_name resource but that is at least documented in the README.
This is due to be fixed in an upcoming Google provider for terraform: https://github.com/terraform-providers/terraform-provider-google/issues/5898