[WFCORE-6801] Use maven.compiler.release to ensure the desired binary…

[yersan]

… compilation level

It also removes the maven.compiler.target and maven.compiler.source properties since they will be replaced by the --release option supported by the maven compiler plugin 3.6. The maven compiler plugin version is inherited from the jboss-parent maven module, so maven.compiler.release will be available for the project. Behind scenes, the maven compiler plugging should be ignoring the --target and --source options when --release is in place since target/source is not compatible with release

Jira issue: https://issues.redhat.com/browse/WFCORE-6801

[yersan]

Thanks @jamezp