Update logging.md for 3.5+ to show zap as default

[jmhbnz]

As outlined in the blog announcing release 3.5[1] etcd now uses zap by default so we should update logging.md to reflect that from v3.5 onward:

The migration to structured logging is complete. etcd now defaults to zap logger that has a reflection-free, zero-allocation JSON encoder. We have deprecated capnslog that logged with reflection-based serialization.

[1]https://etcd.io/blog/2021/announcing-etcd-3.5/

Fixes #605

[jmhbnz]

Hey @spzala, @ahrtr - This one is a pretty small fix for our logging docs, can you please review?

[ahrtr]

LGTM

Thanks @jmhbnz

cc @spzala

[spzala]

Good catch. Thanks @jmhbnz