Migrate iOS default backend to MetalAngle

The @Berstanio RoboVM MetalAngle is now pretty well tested in production apps for quite some time and has proven to be stable and performant. It has several advantages such as fixing issues on arm64 Simulators or uncapping 60 fps hard limit and, of course, being more future-proof.

The idea would be to rename the existing gdx-backend-robovm-metalangle artifact to be the new default gdx-backend-robovm and the current OpenGL based gdx-backend-robovmartifact to become gdx-backend-robovm-opengl. This way the change would be transparent and only very specific users who want to keep the OpenGL backend would need to make a change to their projects.

Related discussion here https://github.com/libgdx/libgdx/pull/7372.