Upstream bug. Re-enable example once the bug is fixed

Due to upstream bug [28306](https://github.com/hashicorp/terraform-provider-google/issues/28306), disabled [code ](https://github.com/GoogleCloudPlatform/terraform-google-cloud-armor/blob/85912eb4a6080e81a8282d22dcb13fc32f8d480f/examples/global-backend-security-policy-enterprise/main.tf#L49)
Re-enable the code once the upstream bug is fixed.