chore: v24 upgrade guide (#1467)

docs/upgrading_to_v24.0.md

@@ -0,0 +1,62 @@
+# Upgrading to v24.0
+The v24.0 release of *kubernetes-engine* is a backwards incompatible
+release.
+
+### master_global_access_enabled in GA private-cluster module
+
+`master_global_access` is now supported in GA private-cluster module and defaults to true. To opt out, set `master_global_access_enabled` to `false`.
+
+```diff
+  module "gke" {
+-   source           = "terraform-google-modules/kubernetes-engine/google//modules/private-cluster"
+-   version          = "~> 23.0"
++   source           = "terraform-google-modules/kubernetes-engine/google//modules/private-cluster"
++   version          = "~> 24.0"
+...
++   master_global_access_enabled  = false
+}
+```
+
+### Update variant random ID keepers updated
+
+The v24.0 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.
+
+1. Perform a `terraform plan` as normal, identifying the `random_id` resources changing and the new `max_pods_per_node` and `pod_range` attributes
+```tf
+      ~ keepers     = { # forces replacement
+          + "max_pods_per_node"           = ""
+          + "pod_range"                   = ""
+            # (19 unchanged elements hidden)
+        }
+        # (2 unchanged attributes hidden)
+    }
+```
+2. Pull the remote state locally: `terraform state pull > default.tfstate`
+1. Back up the original remote state: `cp default.tfstate original.tfstate`
+1. Edit the `random_id` resources to add in the new `max_pods_per_node` and `pod_range` attributes from the `terraform plan` step
+```diff
+"attributes": {
+            "b64_std": "pool-02-vb4=",
+            "b64_url": "pool-02-vb4",
+            "byte_length": 2,
+            "dec": "pool-02-48574",
+            "hex": "pool-02-bdbe",
+            "id": "vb4",
+            "keepers": {
+            ...
+              "taints": "",
++             "max_pods_per_node": "",
++             "pod_range": ""
+            },
+            "prefix": "pool-02-"
+          }
+```
+5. Bump the serial number at the top
+1. Push the modified state to the remote `terraform state push default.tfstate`
+1. Confirm the `random_id` resource no longer changes (or the corresponding `nodepool`) in a `terraform plan`
+
+### Minimum Google Provider versions
+
+Minimum Google Provider versions have been updated to `4.36.0`.