Prepare for POSTGRES_ENABLED false 16.0 breaking change

What is this about?

We're deprecating installing the in-cluster postgresql by default: gitlab-org/gitlab#343988 (closed)

We don't want to break existing installations when we change the template to POSTGRES_ENABLED: false.

Proposal

As suggested by @hfyngvason:

Inject DATABASE_URL if POSTGRES_ENABLED has no value and POSTGRESQL_RELEASE_NAME is found in the cluster