Refactor createSetHeader for cleaner syntax and improved readability

[Ayoub-Mabrouk]

• Replaced var with destructured parameter defaults (digits, header, suffix)
• Used const and let for better scope and immutability
• Simplified conditional checks and reduced code redundancy

[UlisesGascon]

There is a PR that should solve the CI issues: https://github.com/expressjs/response-time/pull/33