Mark min_cpu_platform as computed in instance as it is returned

[slevenick]

This field seems to be returned by the API now, causing diffs if a user has not specified it

Fixes: hashicorp/terraform-provider-google#4975

Release Note Template for Downstream PRs (will be copied)

`google_compute_instance`: marked `min_cpu_platform` as computed so if it is not specified it will not cause diffs

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 2949c7a.

Pull request statuses

WARNING: The following files changed in commit 2949c7a may need corresponding changes in third_party/validator:

• third_party/terraform/resources/resource_compute_instance.go

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1429
depends: hashicorp/terraform-provider-google#4980

[danawillow]

Looks fine, though I'm confused why our tests didn't all fail. Any idea?

[slevenick]

Tests didn't fail because this field is only set for certain machine types, specifically I've seen it come back on when the instance had a machine type of n2-standard-4. I believe this is because the n2 type only works on Cascade Lake architecture at the moment, so it somewhat makes sense to have that field set.