Update README.md to state support for Fastify 5.x

[cwolter97]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

Current README states:

Fastify 4.x. See @fastify/http-proxy v7.x for Fastify 3.x compatibility.

Update this to

Fastify 5.x. See @fastify/http-proxy v9.x for Fastify 4.x compatibility.

or similar

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue?

[cwolter97]

Hey @mcollina, PR created #381