Document how protocol version changes can affect rolling upgrades

[GeoffMontee]

There was a protocol version change in Galera 25.3.23. See here:

https://jira.mariadb.org/browse/MDEV-18944

The Galera documentation should probably contain some notes about how protocol version changes can affect rolling upgrades. I don't see any mentions here:

http://galeracluster.com/documentation-webpages/upgrading.html

I've added the following note to the MariaDB documentation:

Note that when upgrading the Galera wsrep provider, sometimes the Galera protocol version can change. The Galera wsrep provider should not start using the new protocol version until all cluster nodes have been upgraded to the new version, so this is not generally an issue during a rolling upgrade. However, this can cause issues if you restart a non-upgraded node in a > cluster where the rest of the nodes have been upgraded.

https://mariadb.com/kb/en/library/upgrading-between-minor-versions-with-galera-cluster/