You're not even using MySQL at all. You're using MariaDB, a fork of MySQL.
That being said, it should maintain compatibility with MySQL so I'm not really sure what the issue is about.
It has something to do with the following SQL instruction that gets called when initializing PDO by using the "PDO::MYSQL_ATTR_INIT_COMMAND" constant:
You can try typing that for yourself on the SQL tab on phpmyadmin and it should give you a similar error.
For some reason, you seem to not have the utf8 encoding available on your database or MariaDB uses some different way to define it.
Not sure if it's a MariaDB configuration issue or not but maybe somebody else here that is using MariaDB and can shed some more light into this.