Backport "Remove deprecated templates endpoints in V4"

Do not reference /api/:version/dockerfiles/:key deprecated endpoint. Backporting gitlab-ce!8853.

assigned to @rymai

@oswaldo Why does this MR only include the change to app/assets/javascripts/api.js?

Probably because the only conflicting file was this one?

Conflicting files:
  - app/assets/javascripts/api.js:11

– https://gitlab.com/gitlab-org/gitlab-ce/builds/9791661

added ~481018 label

changed milestone to %9.0

@rymai Exactly, it's the only conflict generated by CE MR.

merged