[virtio 1.3] Change freeze has started

As outlined in  https://lists.oasis-open.org/archives/virtio/202304/msg00036.html  ff., as part of the release process for the 1.3 version of the virtio spec, we have now entered change freeze. This means that the only changes to the main virtio branch will now be those needed for the 1.3 release (administrative and editorial.) We hope to have something ready for vote in August.  For any changes for the next virtio release, please target the virtio-1.4 branch, which will eventually merge back into the main development branch once 1.3 has been released. (This next release might not be called 1.4, but that should be inconsequential here.)  virtio 1.3 timeline:  Development  July 3rd: feature freeze (github issue and change on list)

August 2nd: change freeze (all voted upon)  virtio-next development branch opens
<---- we are here  August: prepare draft  August 31st (latest): draft voted upon by TC  September: public review period  October: addtl public review period, if needed  October (November): 1.3 released  merge virtio-next development branch