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

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 <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-id: 20180522104000[email protected]