Remove 5.7 entirely (EOL)

See https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-44.html

> MySQL 5.7.44 is the final release of the MySQL 5.7 series.
>
> MySQL 5.7 users are encouraged to upgrade to the latest MySQL 8.0 or MySQL Innovation release.

Author: tianon