Move Auto DevOps to Cloud Native Buildpacks by default instead of Herokuish

Problem to solve

Herokuish is essentially deprecated in favor of CNBs and will soon become unsupported (no timeline).

With !28165 (merged), we have initial opt-in support for CNBs in Auto Build. Besides lack of customer feedback from the opt-in support, there are two currently known problems with the defaults for all of Auto DevOps:

1. We lack an alternative for Auto Test #212689 (closed)
2. Switching the defaults is a breaking change because /bin/herokuish is no longer present in the resulting image. This makes CNBs incompatible with DB_MIGRATE and DB_INITIALIZE commands, as well as e.g. worker deployments, which typically require application context using /bin/herokuish procfile exec $COMMAND.

Proposed solution

1. In GitLab %14.0, switch Auto Build to use Cloud Native Buildpacks by default
2. Leave Herokuish as an option in Auto Build
3. Leave Auto Test alone