Bump mysql-connector-java in /sparkjava-mysql/backend

Bumps [mysql-connector-java](https://github.com/mysql/mysql-connector-j) from 8.0.11 to 8.0.16.
- [Release notes](https://github.com/mysql/mysql-connector-j/releases)
- [Changelog](https://github.com/mysql/mysql-connector-j/blob/release/8.0/CHANGES)
- [Commits](https://github.com/mysql/mysql-connector-j/compare/8.0.11...8.0.16)

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