Add `on_before_phased_restart` lifecycle event
What does this MR do?
This introduces .on_before_phased_restart method.
Unicorn/Puma Cluster: This will be called before a graceful
shutdown of workers starts happening.
This is called on master process.
References
Based on: !17953 (merged) and omnibus-gitlab!3650 (merged)
Part of: #30201 (closed) !17962 (merged)
Related to: #30037 (closed)
Does this MR meet the acceptance criteria?
Conformity
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