Rename method to not clash with #cluster

We will introduce #cluster at a later stage.
Also deprecate this method

app/models/deployment.rb

@@ -84,7 +84,8 @@ class Deployment < ApplicationRecord
     Commit.truncate_sha(sha)
   end
 
-  def cluster
+  # Deprecated - will be replaced by a persisted cluster_id
+  def deployment_platform_cluster
     environment.deployment_platform&.cluster
   end
 

lib/gitlab/ci/build/prerequisite/kubernetes_namespace.rb

@@ -21,7 +21,7 @@ module Gitlab
           private
 
           def deployment_cluster
-            build.deployment&.cluster
+            build.deployment&.deployment_platform_cluster
           end
 
           def kubernetes_namespace

spec/lib/gitlab/ci/build/prerequisite/kubernetes_namespace_spec.rb

@@ -23,7 +23,7 @@ describe Gitlab::Ci::Build::Prerequisite::KubernetesNamespace do
         let(:cluster) { create(:cluster, :group) }
 
         before do
-          allow(build.deployment).to receive(:cluster).and_return(cluster)
+          allow(build.deployment).to receive(:deployment_platform_cluster).and_return(cluster)
         end
 
         it { is_expected.to be_truthy }
@@ -55,7 +55,7 @@ describe Gitlab::Ci::Build::Prerequisite::KubernetesNamespace do
 
       context 'and no cluster to deploy to' do
         before do
-          expect(deployment.cluster).to be_nil
+          expect(deployment.deployment_platform_cluster).to be_nil
         end
 
         it { is_expected.to be_falsey }
@@ -73,7 +73,7 @@ describe Gitlab::Ci::Build::Prerequisite::KubernetesNamespace do
       let(:cluster) { create(:cluster, :group) }
 
       before do
-        allow(build.deployment).to receive(:cluster).and_return(cluster)
+        allow(build.deployment).to receive(:deployment_platform_cluster).and_return(cluster)
       end
 
       it 'creates a kubernetes namespace' do
@@ -90,7 +90,7 @@ describe Gitlab::Ci::Build::Prerequisite::KubernetesNamespace do
 
     context 'completion is not required' do
       before do
-        expect(deployment.cluster).to be_nil
+        expect(deployment.deployment_platform_cluster).to be_nil
       end
 
       it 'does not create a namespace' do

spec/models/deployment_spec.rb

@@ -380,12 +380,12 @@ describe Deployment do
     end
   end
 
-  describe '#cluster' do
+  describe '#deployment_platform_cluster' do
     let(:deployment) { create(:deployment) }
     let(:project) { deployment.project }
     let(:environment) { deployment.environment }
 
-    subject { deployment.cluster }
+    subject { deployment.deployment_platform_cluster }
 
     before do
       expect(project).to receive(:deployment_platform)