JS: Update Fastify tld

[Fdawgs]

Fastify moved from fastify.io to fastify.dev at the end of 2023.

fastify.io currently redirects to fastify.dev, but this may change in the future. :)

[Copilot]

Pull Request Overview

This PR updates documentation references for Fastify to reflect the new tld by replacing "fastify.io" with "fastify.dev".

• Updated URL in Fastify documentation comments
• Ensured consistency in referencing Fastify endpoints

Comments suppressed due to low confidence (2)

javascript/ql/lib/semmle/javascript/frameworks/Fastify.qll:2

• The documentation URL has been updated to fastify.dev. Verify that all related documentation references are correctly updated for consistency.

 * Provides classes for working with [Fastify](https://www.fastify.dev/) applications.

javascript/ql/lib/semmle/javascript/frameworks/Fastify.qll:9

• Ensure that this updated URL matches the intended new reference for Fastify throughout the module.

 * Provides classes for working with [Fastify](https://www.fastify.dev/) applications.