Use Puma `nakayoshi_fork`

⚠ I'd prefer to sync the merge of the listed 4 MRs. It shouldn't block the review/approvals though 🙂

What does this MR do?

• Relates to gitlab-org/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-org/gitlab!48897 (merged), gitlab-org/gitlab#292918 (closed)) : we decided to keep it for a few more release cycles

Related issues

gitlab-org/gitlab#288042 (closed) gitlab-org/gitlab#292918 (closed)

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

• gitlab-org/gitlab!51467 (merged)
• gitlab-org/omnibus-gitlab!4914 (merged)
• !581 (merged)
• gitlab-org/gitlab-development-kit!1758 (merged)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes - it should be enough to check that the webserver is starting without any issues
• Documentation created/updated
• [-] Integration tests added to GitLab QA
• [-] The impact any change in container size has should be evaluated