docs: deprecate BaseConnection::$strictOn

This property is only for MySQL, and there does not appear to be
any other databases that will require this property in the future.
So it should be in MySQLi\Connection.

Author: kenjis

system/Database/BaseConnection.php

@@ -181,7 +181,9 @@ abstract class BaseConnection implements ConnectionInterface
      *
      * Whether we're running in strict SQL mode.
      *
-     * @var bool
+     * @var bool|null
+     *
+     * @deprecated 4.5.0 Will move to MySQLi\Connection.
      */
     protected $strictOn;