Specify GKE master version less precisely

Cluster creation breaks when the version in min_master_version is no
longer supported by GKE. min_master_version was added when we had a
requirement on the Kubernetes version in GKE, but we can't remove it
without preventing deployment to existing projects.

Specifying the version less precisely buys us time but will still break
occasionally.

b/119610519

Change-Id: I28b5b33540cddd7c7398b160fd577981110f2e88
GitOrigin-RevId: b1f67e4

src/bootstrap/cloud/terraform/cluster.tf

@@ -1,7 +1,7 @@
 resource "google_container_cluster" "cloud-robotics" {
   name               = "cloud-robotics"
   zone               = "${var.zone}"
-  min_master_version = "1.11.5-gke.5"
+  min_master_version = "1.11"
   enable_legacy_abac = true
   depends_on         = ["google_project_service.container"]