main.tf: don't ignore changes in name

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.