QMYSQL plugin no longer works with older supported versions of MySQL/MariaDB

The fix for QTBUG-101680 involves checking for the MYSQL_TYPE_JSON enum. Supporting this enum requires MySQL 5.7.9 or higher:

• https://dev.mysql.com/doc/relnotes/connector-cpp/en/news-1-1-7.html
• https://stackoverflow.com/questions/36931705/how-to-solve-make-mysql-connector-c-1-1-7-error

However, our documentation says that we support older versions:

• MySQL 5.0 for Qt 5.15 (https://doc.qt.io/qt-5/sql-driver.html )
• MySQL 5.6 for Qt 6.x (https://doc.qt.io/qt-6/sql-driver.html )