qapi: Change "since 2.13" annotations to "since 3.0"

We're going to make the next release be 3.0, not 2.13; change
the annotations in our json appropriately.

Changes produced with
  sed -i -e 's/2\.13/3.0/g' qapi/*.json

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180522104000.9044-2-peter.maydell@linaro.org