The content in the porting guides are closer to a changelog than
a porting guide. At this point, it is easier for maintainers and
contributors to write in a changelog than a guide. This change
should help with readability and is closer to the usage of
"Changes" in documentation.

Part of a rename in other submodules.

Task-number: QTBUG-87156
Change-Id: Ib59de7976fae19e7b39962e80161df6628e4070b
Reviewed-by: Paul Wicking <paul.wicking@qt.io>