Since Fastify v5.5.0, there is the following deprecation warning:
> [FSTDPE022] FastifyWarning: The router options for maxParamLength
property access is deprecated. Please use "options.routerOptions"
instead for acce
ssing router options. The router options will be removed in `fastify@6`.

Related fastify/fastify#6282,
fastify/fastify#5985

## 🎯 Changes

Update the Fastify adapter documentation to fix the deprecation warning.

Should we put the 2 possible ways for both v5.4 and prior Fastify
versions, and for v5.5.0 and above, or it's fine to only put the
"latest" way (like how it is done currently in this PR)?

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## ✅ Checklist

- [x] I have followed the steps listed in the [Contributing
guide](https://github.com/trpc/trpc/blob/main/CONTRIBUTING.md).
- [x] If necessary, I have added documentation related to the changes
made.
- [x] I have added or updated the tests related to the changes made.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- Documentation
- Updated Fastify adapter docs to show maxParamLength is now configured
under routerOptions (default 5000).
  - Clarified that server initialization and plugin setup are unchanged.
- Confirmed no changes to public APIs and guided users to configure
maximum URL parameter lengths in the new location.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->