BREAKING CHANGE: Drop legacy PostgreSQL support

This commit drops the legacy PostgreSQL (v0.7.2) support in v2 auto-deploy-image. See #128 (closed)

Users can still migrate their data between channel 1 and 2 in v1 auto-deploy-image.

Today, there are 41 projects on gitlab.com that still uses channel 1:

[ gprd ] production> Ci::Variable.where(key: 'AUTO_DEVOPS_POSTGRES_CHANNEL').select { |v| v.value == '1' 
}.count
=> 41

We should announce that these projects migrate to channel 2 until GitLab 14.0.

But even if it's overdue, they can still migrate with v1 auto-deploy-image.

Documentation MR => gitlab-org/gitlab!47874 (merged)

Close #128 (closed)