Upgrade fastify dependency to V5

[Sevi7]

Is your feature request related to a problem? Please describe. A project that uses fastify V5 cannot register the imported inngest/fastify plugin because it uses fastify V4. I'm trying to use the integration between Clerk and Inngest, and @clerk/fastify requires fastify V5.

Describe the solution you'd like inngest/fastify to use fastify V5