Add GKE support for threads_per_core option

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[X] Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
[X] Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
[X] Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
[x] Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
[X] Read the Release Notes Guide before writing my release note below.

Note: unit tests seem to be broken. I was able to run make test for generated code as follows:

$ make test
$ make fmt
$ make test
...
google/resource_bigtable_app_profile.go:468:9: undefined: bigtableadmin
google/resource_compute_subnetwork.go:39:22: undefined: cidr
...
$ rm google/resource_bigtable_app_profile.go google/resource_bigtable_app_profile_test.go google/resource_compute_subnetwork.go google/resource_compute_subnetwork_test.go
$ sed -i '/resourceBigtableAppProfile/d' google/provider.go
$ sed -i '/resourceComputeSubnetwork/d' google/provider.go
$ make test

Release notes

container: added support for `advanced_machine_features.threads_per_core`

Derived from https://github.com/GoogleCloudPlatform/magic-modules/pull/6808

GoogleCloudPlatform / terraform-validator

Add GKE support for threads_per_core option #1493