Update Fastify Dependency to v3

[henriqueleite42]

Hello!

Thanks for your package, it's very useful!

Since fastify released version 3 more than 1 year ago, I would like to know if it would be possible to update the package dependency to use version 3.

This is mainly due to problems running serlerss package, when it runs the command npm ls -prod -json -depth=1 and finds incompatibility between the version used by the package aws-serverless-fastify (v2 .6.x) is the version used in the local project (v3.22.x).

From the migration guide, I imagine that this change could cause a lot of problems for those who are using v2, but eventually it will have to happen, and the longer we postpone it, the worse it will be.

Thanks again, and I await your feedback.