chore(kad): rename substreams_timeout

[elenaf9]

Description

#6015 made the timeout for outbound substreams in kademlia configurable by introducing a outbound_substreams_timeout config option.
There is currently an open PR #6009 that plans to also apply that timeout to inbound substream, in which case we probably want toe rename the config option to substreams_timeout (without the prefix).
Because we plan to release soon and might not get #6009 in in-time, I propose that we already do the renaming now, to avoid breaking the API again with the next release.

Notes & open questions

I directly changed the initial changelog entry of #6009 to avoid confusion. Is that okay or should a add a separate changelog entry for the renaming?

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

[jxs]

Thanks Elena! LGTM left just one comment

[jxs]

LGTM thanks!