Fix deployments for Kubernetes service templates

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27687

app/models/deployment.rb

@@ -85,7 +85,11 @@ class Deployment < ApplicationRecord
   end
 
   def cluster
-    project.deployment_platform(environment: environment.name)&.cluster
+    platform = project.deployment_platform(environment: environment.name)
+
+    if platform.present? && platform.respond_to?(:cluster)
+      platform.cluster
+    end
   end
 
   def execute_hooks

changelogs/unreleased/60821-deployment-jobs-broken-as-of-11-10-0.yml

+---
+title: Fix Kubernetes service template deployment jobs broken as of 11.10.0
+merge_request: 27687
+author:
+type: fixed

spec/models/deployment_spec.rb

@@ -400,6 +400,12 @@ describe Deployment do
       it { is_expected.to be_nil }
     end
 
+    context 'project uses the kubernetes service for deployments' do
+      let!(:service) { create(:kubernetes_service, project: project) }
+
+      it { is_expected.to be_nil }
+    end
+
     context 'project has a deployment platform' do
       let!(:cluster) { create(:cluster, projects: [project]) }
       let!(:platform) { create(:cluster_platform_kubernetes, cluster: cluster) }