qapi/net.json: Fix the version number of the "vlan" removal

"vlan" will be dropped in 2.13, not in 2.12. And while we're at it,
use the better wording "dropped in" instead of "removed with" (also
for the "dump" removal).

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>