A few years ago, GitLab made the decision to switch to use structured
logging. The flag UNSTRUCTURED_RAILS_LOG was then added to keep
structured and unstructured log in use by lograge. Nowadays, nobody
should be using unstructured logging at GitLab, and this flag will be
now disabled by default.

Eventually, this flag will be removed
completely when confirmed that it's not impacting any deployment.

Changelog: deprecated