Revisit the logic to set the java source/target compatibility

[jdneo]

Full context can be found here: redhat-developer/vscode-java#3721

In short, simply check the compilation options may have some issue for some cases, An example is the project in the linked issue.

User can set the source level to 17, and target level to 8. Meanwhile, use [--release, 8] as the compilation arguments. Normally, this should fail during compilation. But the project uses a annotation processor to make it work.

Which says, maybe we should just simply get the source compatibility from the property, not infer it from the compilation options?

@Arthurm1, WDYT?

[jdneo]

Fixed by #188