Update GitLab-managed helm to 2.16.6

Helm 2.16.6 is a bugfix release and should improve the reliability of
operations on GitLab-managed apps.

See https://github.com/helm/helm/compare/v2.16.3...v2.16.6 for a list of
changes.

changelogs/unreleased/update-gitlab-managed-helm-to-2-16-6.yml

+---
+title: Update GitLab-managed helm from 2.16.3 to 2.16.6, improving the reliability
+  of GitLab's Kubernetes integration
+merge_request: 30067
+author:
+type: changed

lib/gitlab/kubernetes/helm.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module Kubernetes
     module Helm
-      HELM_VERSION = '2.16.3'
+      HELM_VERSION = '2.16.6'
       KUBECTL_VERSION = '1.13.12'
       NAMESPACE = 'gitlab-managed-apps'
       NAMESPACE_LABELS = { 'app.gitlab.com/managed_by' => :gitlab }.freeze

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

@@ -32,7 +32,7 @@ describe Gitlab::Kubernetes::Helm::Pod do
       it 'generates the appropriate specifications for the container' do
         container = subject.generate.spec.containers.first
         expect(container.name).to eq('helm')
-        expect(container.image).to eq('registry.gitlab.com/gitlab-org/cluster-integration/helm-install-image/releases/2.16.3-kube-1.13.12')
+        expect(container.image).to eq('registry.gitlab.com/gitlab-org/cluster-integration/helm-install-image/releases/2.16.6-kube-1.13.12')
         expect(container.env.count).to eq(3)
         expect(container.env.map(&:name)).to match_array([:HELM_VERSION, :TILLER_NAMESPACE, :COMMAND_SCRIPT])
         expect(container.command).to match_array(["/bin/sh"])