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

[ctrueden]

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 <artifactId>imagej2</artifactId> 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.

[ctrueden]

In Nov 2023, we decided to adjust the branding of ImageJ/ImageJ2/Fiji as described in this forum thread. In a nutshell: shift away from the name "ImageJ2" in favor of "Fiji" for the core permissively licensed application. In the near term, we will want to rename this project to something like fiji-core.