Specify target Java version to build for

When not specifying the target Java version, the version of the installed JDK is targetted. This leads to errors when trying to execute the generated JAR file on a system with an earlier Java version than the one it was built with. Specify the lowest possible target version according to the README to ensure that the compiled JAR runs on as many systems as possible.

This was reported downstream in Arch Linux as FS#64111.