Consolidate Puma low-level handler

Right now we generate puma.rb in three different places (CNG, Omnibus, and GDK). Anytime we need to tweak a setting or change the low-level handler, we have to file three separate merge requests. For example:

1. CNG: gitlab-org/build/CNG!1556 (merged) (this would fix the issue in production)
2. Omnibus: omnibus-gitlab!7161 (merged)
3. GDK: gitlab-development-kit!3371 (merged)

The Puma DSL defines the lowlevel_error_handler. We might be able to store this in a separate file and provide it as a server option (:lowlevel_error_handler) instead.