fix: set sensitive for gh-runner-gke ca_certificate output

[cjonesy]

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 here.

This PR correctly sets the ca_certificate output as sensitive.

This becomes problematic when using the module from Terragrunt as you'll get the following error:

╷
│ Error: Output refers to sensitive values
│ 
│   on outputs.tf line 29:
│   29: output "ca_certificate" {
│ 
│ 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
╵
ERRO[0069] Terraform invocation failed in /Users/cjones/src/terragrunt-repo/development/sandboxes/cjones/scratch/gke-gha/.terragrunt-cache/HaQJoRguUiWbQzqk_oSxpSvldDk/1kW9xrwM9QZFpZ4HlxpmJGveo_g/modules/gh-runner-gke  prefix=[/Users/cjones/src/terragrunt-repo/development/sandboxes/cjones/scratch/gke-gha] 
ERRO[0069] 1 error occurred:
        * [/Users/cjones/src/terragrunt-repo/development/sandboxes/cjones/scratch/gke-gha/.terragrunt-cache/HaQJoRguUiWbQzqk_oSxpSvldDk/1kW9xrwM9QZFpZ4HlxpmJGveo_g/modules/gh-runner-gke] exit status 1

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[bharathkkb]

/gcbrun

[bharathkkb]

Thanks for the PR @cjonesy

[apeabody]

/gcbrun