Drop 'experimental' references to Puma

What does this MR do?

Puma is being promoted to stable, so we need to remove all references to its former experimental status from code, configuration and docs.

Refs #207445 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content