fix(kad): actually report unsupported protocol

[thomaseizinger]

Description

It turns out, we never actually used ProtocolStatus::NotSupported. Or at least, we never constructed it within the Handler. This didn't get flagged by rustc because

a) we allowed the dead_code lint across libp2p-kad
b) the HandlerEvent enum is public and thus could be constructed by users

Related: #4632.
Related: #4633.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[mxinden]

A story of the power of Rust and how not to make use of it.

Good catch!

Missing a changelog entry, right? Otherwise looks good to me.

[thomaseizinger]

Missing a changelog entry, right?

If only CI would tell me! Oh wait 😁