Update changelog for #12587 to be more accurate (#12663)

#12587 has fallen on the wrong side of the release cutoff to the rest of
the related PRs.

Signed-off-by: Sean Quah <seanq@element.io>

changelog.d/12587.misc

-Add `@cancellable` decorator, for use on endpoint methods that can be cancelled when clients disconnect.
+Log status code of cancelled requests as 499 and avoid logging stack traces for them.

changelog.d/12663.misc

+Log status code of cancelled requests as 499 and avoid logging stack traces for them.