docs: upgrade guide for #2256 (#2261)

Author: apeabody

docs/upgrading_to_v36.0.md

@@ -0,0 +1,26 @@
+# Upgrading to v36.0
+The v36.0 release of *kubernetes-engine* is a backwards incompatible release.
+
+### var.enable_gcfs removed from Autopilot sub-modules
+The variable `enable_gcfs` has been removed from the Autopilot sub-modules. Autopilot clusters that run GKE version `1.25.5-gke.1000` and later use [Image streaming](https://cloud.google.com/kubernetes-engine/docs/how-to/image-streaming).
+
+```diff
+  module "cluster" {
+-   version          = "~> 35.0"
++   version          = "~> 36.0"
+
+-   enable_gcfs = true
+}
+```
+
+### var.logging_variant removed from Autopilot sub-modules
+The variable `logging_variant` has been removed from the Autopilot sub-modules. It is only [applicable](https://cloud.google.com/kubernetes-engine/docs/how-to/adjust-log-throughput) to Standard clusters.
+
+```diff
+  module "cluster" {
+-   version          = "~> 35.0"
++   version          = "~> 36.0"
+
+-   logging_variant = "DEFAULT"
+}
+```