treat the MYSQL_FIELD as read-only

treat the MYSQL_FIELD as read-only

Origin: upstream, https://code.qt.io/cgit/qt/qtbase.git/commit/?id=549ee216fd5bf2b3
Last-Update: 2021-11-13

The MariaDB-connector-c version 3.2 and MariaDB server version 10.6
cooperate to avoid re-transferring the query metadata, so the fact that
we were modifying it was causing it to improperly decode the DATETIME
data types into string, as we had asked. We ended up with a 7-byte
string that was actually the date binary-encoded.

Gbp-Pq: Name mysql_field_readonly.diff