Add information on Rocky 9 login nodes

[stevenvdb]

Since two login nodes have been upgraded to Rocky 9 as well, the documentation page about it needed some updates.

[stevenvdb]

I now realize that the module spider command can also cause confusion, since it cannot fully handle the separation of modules for different operating systems. So if a module is available for rocky8 but not rocky9, module spider will show it can be loaded after loading an appropriate cluster module, but that might be depending on which OS the user is on. I am not sure if there is a clean solution, maybe some more explanation is needed.