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’;
$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!