bpo-42392: Mention loop removal in whatsnew for 3.10 (GH-24256)

@vstinner [noticed on python-dev](https://mail.python.org/archives/list/python-dev@python.org/thread/O3T7SK3BGMFWMLCQXDODZJSBL42AUWTR/) that there is no what's new or porting entry for removal of asyncio ``loop`` parameter. 

This patch adds a basic guide.

Co-Authored-By: Kyle Stanley <aeros167@gmail.com>