set k3s_token output to sensitive

To reduce the risk of accidentally exporting sensitive data that was
intended to be only internal, Terraform requires that any root module
output containing sensitive data be explicitly marked as sensitive, to
confirm your intent.

If you do intend to export this data, annotate the output value as
sensitive by adding the following argument:
    sensitive = true

output.tf

@@ -45,6 +45,7 @@ output "k3s_endpoint" {
 output "k3s_token" {
   description = "The k3s token to register new nodes"
   value       = local.k3s_token
+  sensitive   = true
 }
 
 # Keeping for backward compatibility