add a release note for -zmqpubsequence

Summary:
Adapted from https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.21.0.md

Depends on D10304

Test Plan: Proofreading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10326