make provisioning_model O+C in google_compute_instance_template

[modular-magician]

provisioning_model field was originally added in PR GoogleCloudPlatform/magic-modules#5662
Some of the compute_instance_template related tests start to fail in our nightly tests yesterday https://ci-oss.hashicorp.engineering/buildConfiguration/GoogleCloudBeta_ProviderGoogleCloudBetaGoogleProject/255845?buildTab=tests&status=failed&name=TestAccComputeInstanceTemplate&pager.currentPage=1

Just reached out to the product team, and it seems like it has started to set a default value STANDARD for this provisioning_model field for non-preemptible VMs from this Monday. Therefore, making this field optional computed.

Already added computed to google_compute_instance resource in GoogleCloudPlatform/magic-modules#5698, and now adding this to google_compute_instance_template resource

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Release Note Template for Downstream PRs (will be copied)

compute: fixed permadiff in `google_compute_instance_template.scheduling.provisioning_model`

Derived from GoogleCloudPlatform/magic-modules#5705