AbstractHelpMojo.LS uses System.getProperty instead of System.lineSeparator()

[elharo]

In src/main/java/org/apache/maven/plugins/help/AbstractHelpMojo.java:55:

protected static final String LS = System.getProperty("line.separator");

Should use System.lineSeparator() (Java 7+), which falls back to "\n" if the property is missing. DescribeMojoTest inconsistently uses both System.getProperty("line.separator") (line 88) and System.lineSeparator() (line 110).