KAFKA-7481; Add upgrade/downgrade notes for 2.1.x

We seemed to be missing the usual rolling upgrade instructions so I've added them and emphasized the impact for downgrades after bumping the inter-broker protocol version.

Author: Jason Gustafson <jason@confluent.io>

Reviewers: Dong Lin <lindong28@gmail.com>, Ismael Juma <ismael@juma.me.uk>

Closes #5857 from hachikuji/KAFKA-7481