revert to “New in master” in CHANGES.txt

I also cleaned up former headers to be in "New in vX.Y.Z" formats.
We had a little mess:

v4.x             “New in Version X(.Y.Z)”
v5(.1)           “New in vX(.Y)”
v5.2 and higher  “New in X.Y.Z”.

It’s all now unified as “New in vX.(Y(.Z))”.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>