check-news: Ensure release date is within three days

Commit b6ad806f contained a date a month in the future. With this patch
all release dates in NEWS may at most be three days into the future
(assuming the build machine's clock is correct).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>