terraform: only set confidential_instance_type if cc_technology is SEV_SNP

[daniel-weisse]

Context

Upgrades of existing GCP SEV-ES clusters are failing because setting confidential_instance_type makes Terraform want to re-create the instance templates, which does not work because they are still in use by instances.

Proposed change(s)

• Only set confidential_instance_type if cc_technology is SEV_SNP
  • When this feature is introduced into the mainline GCP Terraform provider, we will likely have to introduce some form of migration, but I would hold of on that until the provider maintainers decide on how this will look like

Related issue

• Fixes https://github.com/edgelesssys/issues/issues/545

Checklist

• Run the E2E tests that are relevant to this PR's changes
  • GCP SEV-ES upgrade test
  • GCP SEV-SNP test

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