The latest version of the Build Helper Maven plugin (1.10 at the moment) requires Java 1.7, which broke my build. The plugin documentation page only shows Java version requirement for the latest version.
Looking at Build Helper Maven Plugin versions, here
version 1.9.1 points to parent POM org.codehaus.mojo.mojo-parent version 33, which still sets its Java target at 1.5 (via the property mojo.java.target).
version 1.10 points to parent POM org.codehaus.mojo.mojo-parent version 38, which sets its Java target at 1.7
Hence, Build Helper Maven Plugin 1.9.1 is the latest version compatible with Java 6. From that version on, Java 7 is the default target.