chore: move mongodb as peer dependency

[rtritto]

Fastify v5 is released

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

[rtritto]

FYI @gurgunday

[jsumners]

Thank you, but peer dependencies cause more problems than they solve. We prefer to avoid them.

[rtritto]

As standard, all plugins use peer dependencies because a specific version can be selected.

[Eomm]

In these duplicates, you can find the workaround:

fastify/fastify-swagger#220
fastify/fastify-static#159

The decision has been to don't add it as peerDependencies.

Since npm v10 has been released, we should re-evaluate it: would you like to analyze if it is doable?