I see https://github.com/forge/core/blob/master/project-model-maven/src/main/java/org/jboss/forge/maven/facets/MavenJavaSourceFacet.java#L182 has hardcoded source/target versions for the maven-compiler-plugin configuration

IMHO, these (along the encoding) should be overrideable via CLI options. Also, the default version should be inferred from the currently running JVM. One caveat though, is that would make forge build tests dependent on the running JVM.