Okay so the problem is that my config.php file needed to be changed to reflect the change from MySQL to MariaDB.

In config.php I changed this:

$CFG->dbtype    = ‘mysql’;

To this:

$CFG->dbtype    = ‘mariadb’;

And this allowed upgrade to continue.

Another question though…

In this article: https://docs.moodle.org/26/en/MariaDB
it shows that the following line should also be added to config.php:

$CFG->dblibrary = ‘native’;

This is going way back to v2.6 so is this still necessary, and should I add it?

I am going to assume that YES I should!