Simplify `ConnectionHandler` trait by removing as many associated types as possible

Simplify the ConnectionHandler interface. This is one of the first interfaces users get in touch with when the implement their own protocols. Making it as simple as possible without removing functionality should be the goal. All but {To,From}Behaviour should be removed and we should directly hand the negotiated stream to the ConnectionHandler without any form of "upgrade".

### Sub-epics
- [ ] https://github.com/libp2p/rust-libp2p/issues/4306
- [ ] https://github.com/libp2p/rust-libp2p/issues/3268
- [ ] https://github.com/libp2p/rust-libp2p/discussions/4585
- [ ] https://github.com/libp2p/rust-libp2p/issues/3591

### Use `ReadyUpgrade` everywhere
- [ ] https://github.com/libp2p/rust-libp2p/issues/4075
- [ ] https://github.com/libp2p/rust-libp2p/pull/4563
- [ ] https://github.com/libp2p/rust-libp2p/pull/3914
- [ ] https://github.com/libp2p/rust-libp2p/issues/4649
- [ ] refactor(dcutr): use `ReadyUpgrade` for `{In,Out}boundUpgrade`
- [ ] refactor(gossipsub): use `ReadyUpgrade` for `{In,Out}boundUpgrade`

### Migration plan
- [ ] https://github.com/libp2p/rust-libp2p/issues/4307
- [ ] https://github.com/libp2p/rust-libp2p/issues/4521
- [ ] https://github.com/libp2p/rust-libp2p/issues/4697
- [ ] https://github.com/libp2p/rust-libp2p/issues/4790

Start by locating the ConnectionHandler interface and reading the linked sub-epics and migration-plan issues. Done means simplifying the interface by retaining only To and FromBehaviour associated types and handing the negotiated stream directly to the handler without an upgrade.