docs: require TPGv6 upgrade doc (terraform-google-modules#2164)

Author: apeabody

docs/upgrading_to_v35.0.md

@@ -0,0 +1,5 @@
+# Upgrading to v35.0
+The v35.0 release of *kubernetes-engine* is a backwards incompatible release.
+
+### Google Cloud Platform Provider upgrade
+The Terraform Kubernetes Engine Module now requires version 6 of the Google Cloud Platform Providers. See the [Terraform Google Provider 6.0.0 Upgrade Guide](https://registry.terraform.io/providers/hashicorp/google/latest/docs/guides/version_6_upgrade) for more details.