fix: sensitive gh-runner-gke ca_certificate output

The gh-runner-gke module depends on terraform-google-kubernetes-engine/beta-private-cluster
and passes through some of it's outputs. However the `ca_certificate` output is not
properly tagged as sensitive = true to match it's upstream counterpart.

This commit correctly sets the ca_certificate output as sensitive.

modules/gh-runner-gke/outputs.tf

@@ -28,6 +28,7 @@ output "client_token" {
 
 output "ca_certificate" {
   description = "The cluster ca certificate (base64 encoded)"
+  sensitive   = true
   value       = module.runner-cluster.ca_certificate
 }