Properly define the JAVA API of the artifacts uploaded to maven

[NilsHasenbanck4758]

Hi,

When we upgraded flummi 7 to 8, we saw that the class files uploaded to maven are compiled with JDK17 as the target (major version: 57). Since our Jenkins CI is still running on JDK11, we can't upgrade to the version 8 of flummi (we can't even take the route to include the project as a source project, since it depends on the maven plugin, which is not available anymore in gradle 7.*).

I think there is a configuration for Idea to set the JDK version to 1.8. But I don't think those values are used when building and uploading to maven.

Could we maybe properly configure the version and also upload a new version to maven?

For example, if you still want to target JDK8:

    sourceCompatibility = "1.8"
    targetCompatibility = "1.8"