Enable frozen string for kubernetes specs

Partially addresses:

https://gitlab.com/gitlab-org/gitlab/issues/27703

spec/lib/gitlab/kubernetes/config_map_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Kubernetes::ConfigMap do

spec/lib/gitlab/kubernetes/helm/api_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Kubernetes::Helm::Api do

spec/lib/gitlab/kubernetes/helm/base_command_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Kubernetes::Helm::BaseCommand do

spec/lib/gitlab/kubernetes/helm/init_command_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Kubernetes::Helm::InitCommand do

spec/lib/gitlab/kubernetes/helm/install_command_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Kubernetes::Helm::InstallCommand do

spec/lib/gitlab/kubernetes/helm/pod_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Kubernetes::Helm::Pod do

spec/lib/gitlab/kubernetes/namespace_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Kubernetes::Namespace do