Checklist

• My issue is specific & actionable.
• I am not suggesting a protocol enhancement.
• I have searched on the issue tracker for my issue.

Description

libp2p has deprecated mplex for a long time. The implementation is not being maintained any more, and we're planning to drop it from our transport integration test suite due to its flakiness (read: bugs that we don't feel like debugging).

js-libp2p has had yamux support for a while now, as does rust-libp2p. Would it be a good time to say goodbye to our beloved mplex?

Done criteria

• mplex is not enabled by default
• Changelog specifies that mplex will be deprecated in the next Kubo release, how to reenable it, and points them that we'd like their feedback if they still require mplex. We should point to fully remove mplex #10069
• (nice to have) Log a warning if someone has mplex enabled.