docs: add upgrading_to_v38.0.md (#2406)

Author: apeabody

docs/upgrading_to_v38.0.md

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+# Upgrading to v38.0
+The v38.0 release of *kubernetes-engine* is a backwards incompatible release.
+
+### Google Cloud Platform Provider upgrade
+The Terraform Kubernetes Engine Module now requires version 6.42 or higher of the Google Cloud Platform Providers.
+
+### Update variant random ID keepers updated
+
+The v38.0 release updates the keepers for the update variant modules. This will force a recreation of the nodepools. To avoid this, it is possible to edit the remote state of the `random_id` resource to add the new attributes.