docs: replace uses of variadic .listen function

[blimmer]

The documentation references .listen(<port>) in several places, but this variadic function is deprecated in Fastify v4 and throws this warning:

(node:254) [FSTDEP011] FastifyDeprecation: Variadic listen method is deprecated. Please use ".listen(optionsObject)" instead. The variadic signature will be removed in fastify@5.

Since the current package.json requires Fastify v4+, this should be safe to do:

mercurius/package.json

Line 44 in bbb8b26

"fastify": "^4.0.0",

Deprecation guide: https://github.com/fastify/fastify/blob/2fd918298d55aaab74585c0b71081ad40988b280/docs/Guides/Migration-Guide-V4.md#deprecation-of-variadic-listen-signature

[mcollina]

lgtm