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

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

[netlify[bot]]

✅ Deploy Preview for constellation-docs canceled.

Name	Link
🔨 Latest commit	87c3f367cd45f530b709effc9eb7ff4718c625d8
🔍 Latest deploy log	https://app.netlify.com/sites/constellation-docs/deploys/66434d0addcec300089b812d