Bump AMDGPU compat to 0.6

[pxl-th]

And drop 0.4

PR Checklist

• Tests are added
• Documentation, if applicable

[pxl-th]

Maybe we should add CUDA/AMDGPU/etc. to Project.toml only if get(ENV, "NNLIB_TEST_CUDA", "false") == "true" with Pkg.add("CUDA")?
This would avoid potential incompatibilities between the two packages.

[CarloLucibello]

Maybe we should add CUDA/AMDGPU/etc. to Project.toml only if get(ENV, "NNLIB_TEST_CUDA", "false") == "true" with Pkg.add("CUDA")?

you suggest we use Pkg.add(...) inside the if blocks in runtests.jl file? We can do that. Let's merge #531 first though.