pom-scijava: update NAR plugin's groupId and version
pom-scijava: update NAR plugin's groupId and version

We no longer use our custom release but rather the second release
candidate of the shiny new nar-maven-plugin on Maven Central with the
shiny new groupId com.github.maven-nar!

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

scijava/scijava-common