Java 8 as minimum

[jorsol]

Maven is starting to move slowly to Java 8: https://issues.apache.org/jira/browse/MJAR-280

Actually, even if right now plexus-archiver targets Java 7, commons-compress 1.21 depends on Java 8 and has to be downgraded in maven-archiver to work with Java 7.

plexus-utils 3.4.x also have moved to Java 8.