Release notes: Maintain 1.20 notes in master, revert history.
* The one in history isn't final yet, which makes maintenance
harder as it would have to be synced between 2 different file
names across 2 different branches.
* For backport commits its easier if the 1.20 file is still in
master so that they can be written together with the commit
(and so that the history is in master as well), as opposed to
merging it in master without notes and then adding them later
on in the (amended) merge version.
Change-Id: I3fa4b88a5845a51e83221d78facaba1ddb3b1acb