Improve support for graceful shutdown on Puma/Unicorn

What does this MR do?

• Support SIGTERM on Puma
• Support SIGQUIT on Unicorn
• Rename on_before_phased_restart to on_before_graceful_shutdown
• Ensure that we allow to run only in Puma Cluster mode
• Ensure that we run graceful shutdown only when doing a shutdown from the loop
• Ensure that we test all conditions and signal handling

Fixes and improves handling for #34868 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team