Update to if-watch v2.0.0 in mdns

## Description

As it was done in https://github.com/libp2p/rust-libp2p/pull/2813

## Motivation

```
$ fd Cargo.toml --exec rg -H if-watch {}
protocols/mdns/Cargo.toml:if-watch = "1.1.1"
transports/tcp/Cargo.toml:if-watch = "2.0.0"
```

At least it would be great to compile less code.