Rename artifact from net.imagej:imagej to sc.fiji:fiji-core

This artifact (`net.imagej:imagej`) should be renamed to `net.imagej:imagej2` for clarity.

* All downstream code depending on `net.imagej:imagej` (e.g. `example-imagej-command` which should be renamed to `example-imagej2-command`) would need to update their POMs from `imagej` to `imagej2` when they update to pom-scijava 31.0.0.
* All documentation would need updating to show `imagej2` and so forth, rather than `imagej`.
* PyImageJ would need updating to reason about the given version number to `imagej.init` to use `net.imagej:imagej2` when the version is 2.4.0 or later, and `net.imagej:imagej` when it's earlier.