Auto DevOps: Default to version 2 for provisioned Postgres

In #32720 (closed), we added support for Postgres chart 8.2.1 (let's call this Postgres version 2). But Auto DevOps currently defaults to Postgres chart 0.7.1 (Let's call this Postgres version 1).

In GitLab 13.0, we want to change the default from Postgres version 1 to Postgres version 2.

Impact

With #211890 (closed) implemented, the impact on pipelines utilizing the legacy option is that the pipeline will refuse to proceed. User will have to additionally opt-in to switch from legacy to new option as this actively deletes the legacy DB.

Impact on GitLab.com

As we will implement a stop and fail pipeline solution, I think this is OK to release on GitLab.com whenever it's ready.

Proposal

1. #211890 (closed)
2. Change the default from Postgres version 1 to Postgres version 2. (precisely, change AUTO_DEVOPS_POSTGRES_CHANNEL from default 1 to 2)
   • This is a safe operation if this is the first Auto DevOps deploy, or they have previously already been using Postgres version 2.
   • Also bump POSTGRES_VERSION to 9.6.16

Follow-up issue

• #217906