Pin the GKE version to 1.18 for cluster creation

Basic authentication was removed for GKE cluster versions 1.19
Our API call to create GKE clusters did not specify a cluster version,
thus the integration started breaking as soon as Google defined that
1.19 is the default version.

The quickest solution is to pin the k8s version to 1.18.

Changelog: fixed

lib/google_api/cloud_platform/client.rb

@@ -13,6 +13,10 @@ module GoogleApi
       LEAST_TOKEN_LIFE_TIME = 10.minutes
       CLUSTER_MASTER_AUTH_USERNAME = 'admin'
       CLUSTER_IPV4_CIDR_BLOCK = '/16'
+      # Don't upgrade to > 1.18 before we move away from Basic Auth
+      # See issue: https://gitlab.com/gitlab-org/gitlab/-/issues/331582
+      # Possible solution: https://gitlab.com/groups/gitlab-org/-/epics/6049
+      GKE_VERSION = '1.18'
       CLUSTER_OAUTH_SCOPES = [
         "https://www.googleapis.com/auth/devstorage.read_only",
         "https://www.googleapis.com/auth/logging.write",
@@ -90,6 +94,7 @@ module GoogleApi
           cluster: {
             name: cluster_name,
             initial_node_count: cluster_size,
+            initial_cluster_version: GKE_VERSION,
             node_config: {
               machine_type: machine_type,
               oauth_scopes: CLUSTER_OAUTH_SCOPES

spec/lib/google_api/cloud_platform/client_spec.rb

@@ -91,6 +91,7 @@ RSpec.describe GoogleApi::CloudPlatform::Client do
         cluster: {
           name: cluster_name,
           initial_node_count: cluster_size,
+          initial_cluster_version: '1.18',
           node_config: {
             machine_type: machine_type,
             oauth_scopes: [