Stop using the Maven 2.x compatibility layer in the XAR Maven plugin

The plugin uses maven-compat APIs (MavenProjectBuilder, ArtifactFactory, MavenProject#createArtifacts, ArtifactResolver), so Maven 3.9 reports that it depends on the deprecated Maven 2.x compatibility layer, which will not be supported in Maven 4.x.

Those APIs are only used by the unxar mojo to also unpack the XAR dependencies of the expanded artifact, which never unpacks anything in practice: the dependencies inherit the test scope and are then excluded by the runtime scope filter. That code is removed (the documented behavior of unxar, expanding the XAR file, does not change), along with the maven-compat dependency.