swarm: Remove `{In,Out}boundUpgrade`

Whilst working on https://github.com/libp2p/rust-libp2p/pull/4789, I had an epiphany. So far, I was of the opinion that we will remove the UpgradeInfo trait together with the {In,Out}boundUpgrade traits.

Doing this always left a somewhat bad taste because the associated types on UpgradeInfo is what allows the entire SelectUpgrade machinery to work:

https://github.com/libp2p/rust-libp2p/blob/d05d83615963747eabc59c2f4a247b0cec041e6e/core/src/upgrade/select.rs#L43-L68

In other words, building a tree of ConnectionHandlers, getting their protocols and dispatching back to the correct handler only works because of the associated type of UpgradeInfo::Info.

To remove some of the complexity of ConnectionHandlers but still retaining this property, we can simply keep the UpgradeInfo trait and just remove the upgrade step. I made a PoC for this in https://github.com/libp2p/rust-libp2p/pull/4791/commits/310311aed134d747bf7ba31a3bb034d8d0899606. It works actually quite well. I've only adjusted the "ping" protocol here: https://github.com/libp2p/rust-libp2p/blob/310311aed134d747bf7ba31a3bb034d8d0899606/protocols/ping/src/handler.rs#L330-L366

In essence, this now always received a Stream and the protocol is the one that was negotiated. If a user uses ReadyUpgrade, that will always be an instance of StreamProtocol.

Start with core/src/upgrade/select.rs and the ping handler around protocols/ping/src/handler.rs#L330-L366, then compare the proof of concept in PR #4791. Determine the full scope of removing the upgrade step while retaining UpgradeInfo and protocol dispatch; done when the affected upgrade and handler paths are consistently updated and the behavior is validated.