Increase CUDA.jl compat to 5.0.0?

[johnbcoughlin]

Just creating an issue to ask if there are blockers for this to happen. I am using NNlibCUDA.jl to access CUDA bindings for convolution routines, but not using Flux at all.

[ToucheSir]

NNlib itself now has built-in CUDA integration via Julia package extensions. As such, NNlibCUDA is no longer required and more or less deprecated on newer Julia versions.

[MariusDrulea]

If this is the case, I think we shall add a deprecation warning in the readme.