Rename CMake target `nvforest++` to `nvforest`

[hcho3]

Follow RAPIDS convention by renaming the CMake target nvforest++ to nvforest and libnvforest++.so to libnvforest.so.

Related. rapidsai/cuml#8048 (comment), rapidsai/cuml#8011

[bdice]

Please also track renaming libnvforest++.so -> libnvforest.so as part of this. Thank you!

[csadorf]

Please also track renaming libnvforest++.so -> libnvforest.so as part of this. Thank you!

Edited the issue description.