Minimum versions of databases with JSON data type support
Minimum versions of databases with JSON data type support

SQLAlchemy collector requires database JSON data type support
which is fairly new feature and missing in versions older than:

    - MariaDB 10.2
    - MySQL 5.7.8

Let's document that SQLAlchemy collector requirement.

Change-Id: Ifb379a40d90d252d56d4f830e75bbfa08bf66a01
Closes-Bug: #1825693

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