Fix the build by using a valid parent POM

The latest version of net.imagej:pom-imagej is 5.0. The latest
version of sc.fiji:pom-fiji 5.0.2 (which is itself based on
net.imagej:pom-imagej:5.0). Since this project depends only on
net.imagej:ij, it doesn't matter too much which of those two
parents is chosen, as long as the GAV is correct.