Upgrade 'testing-preview' VM in infra

Add 4 CPUs to the VM.

Author: jpraynaud

.github/workflows/ci.yml

@@ -576,7 +576,7 @@ jobs:
           terraform_backend_bucket: hydra-terraform-admin
           google_region: europe-west1
           google_zone: europe-west1-b
-          google_machine_type: e2-highmem-4
+          google_machine_type: e2-standard-8
           google_compute_instance_boot_disk_size: 200
           google_compute_instance_data_disk_size: 250
     environment: ${{ matrix.environment }}