Update for Fastify v5

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Ref: https://github.com/fastify/fastify/issues/5116

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

CC @simoneb

[simoneb]

done and rebased