Use Puma `nakayoshi_fork`

What does this Merge Request do?

• Implements gitlab#288042 (closed).
• Enables nakayoshi_fork by default
• Uses ENV var for better control of the option
• Removes if defined check which was needed prior to Puma 5.1 migration for compatibility (gitlab!48897 (merged), gitlab#292918 (closed))

All config-related MRs for this change (for cross-reference):

• gitlab!51467 (merged)
• omnibus-gitlab!4914 (merged)
• gitlab-org/build/CNG!581 (merged)
• !1758 (merged)

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.