chore: fastify v4 compatibility

[simoneb]

This PR introduces the (non functional) changes needed to make this package work with Fastify v4 (only docs and tests). Notes this uses as merge base the dependabot branch.

Checklist

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

[mcollina]

lgtm

[simoneb]

if somebody could merge this PR then we can check that the dependabot one also works and then do a release

[simoneb]

btw this repo could be under the fastify-plugins team umbrella, and I could do those things myself if it was :)

[jsumners]

This would be under the libraries team, as this isn't really a plugin. I can make that happen and get you on that team. Agreed, @mcollina ?

[simoneb]

oh ok gotcha, I didn't know there was another team for this. I don't mind, I not contributing that often around here anyway

[mcollina]

go for it @jsumners

[jsumners]

Done. I have some work to do with getting some other libraries into the npm org, but this one is there.