UNSTRUCTURED_RAILS_LOG disabled by default

What does this MR do and why?

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.

Part of https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/2145, gitlab-com/gl-infra/scalability#2191 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.