Summary:
I have some pending commits which will debut in GHC 8.8.1,
but we don't yet have release notes for this. This adds them, and
deletes the stale 8.4.1 and 8.4.2 release notes.

Test Plan: Read it

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5025