Deprecate :alpine-helm image tag

What does this MR do and why?

Deprecate :alpine-helm image tag.

That image includes:

• Alpine 3.11 (EOL)
• Ruby 2.4 (EOL)

Impact

These repos have been using this tag:

• https://gitlab.com/gitlab-org/gitlab-omnibus-builder
  • gitlab-omnibus-builder!226 (merged) will get rid of this image tag
• https://gitlab.com/gitlab-org/charts/auto-deploy-app
  • Deprecated and https://gitlab.com/gitlab-org/cluster-integration/auto-deploy-image is not using that tag