feat: Set AUTO_DEVOPS_POSTGRES_CHANNEL to 2 by default

We need to unset the default POSTGRES_VERSION in the CI YAML for this to work out of the box for new deployments.

I.e. this should be followed up with an MR to gitlab-org/gitlab doing the following:

• move the POSTGRES_VERSION variable out of Auto-DevOps.gitlab-ci.yml and into the test job in Test.gitlab-ci.yml
• update auto-deploy-image in both Deploy.gitlab-ci.yml and DAST-Default-Branch-Deploy.gitlab-ci.yml

See gitlab-org/gitlab#210499 (closed)