Use Cloud Native Buildpacks by default in Auto Build

What does this MR do?

Auto Build Image now defaults to Cloud Native Buildpacks instead of Herokuish when no Dockerfile is present. Users depending on the /bin/herokuish binary provided by Herokuish should either change their deployments to use /cnb/lifecycle/launcher instead of /bin/herokuish procfile exec, or opt-out of using Cloud Native Buildpacks, by setting the CI variable AUTO_BUILD_IMAGE_CNB_ENABLED to false.

• auto-build-image release https://gitlab.com/gitlab-org/cluster-integration/auto-build-image/-/releases/v1.0.0
• Issue #212692 (closed)
• Blog post includes the change https://about.gitlab.com/blog/2021/06/04/gitlab-moving-to-14-breaking-changes/

Manual QA

A full pipeline from an example project using the following CI YAML with the latest Rails project template:

include:
  - template: Auto-DevOps.gitlab-ci.yml

variables:
  DB_INITIALIZE: RAILS_ENV=production /cnb/lifecycle/launcher bin/rails db:setup
  DB_MIGRATE: RAILS_ENV=production /cnb/lifecycle/launcher bin/rails db:migrate

build:
  image: registry.gitlab.com/gitlab-org/cluster-integration/auto-build-image:v1.0.0

Screenshots (strongly suggested)

Demonstrates the above rails app with a custom worker launched via /cnb/lifecycle/launcher:

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.