deps(iroh-gossip): Do not depend directly on Quinn (#2678)

## Description

iroh-gossip has a direct dependency on Quinn but does not need it.
Remove this.

## Breaking Changes

none

## Notes & open questions

Please do not merge before #2595 is merged. I'm trying to not modify
that PR any further.

## Change checklist

- [x] Self-review.
- ~~[ ] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.~~
- ~~[ ] Tests if relevant.~~
- ~~[ ] All breaking changes documented.~~

Author: flub