Add [[deprecated]] to cu::Context

[csbnw]

Is your feature request related to a problem? Please describe.
Now that cudawrappers supports HIP, cu::Context only properly works when using CUDA as a backend and are executed as no-ops with HIP.

Describe the solution you'd like
We may want to discourage users from using these functions even with CUDA by adding [[deprecated]] tags.

Describe alternatives you've considered
N.A.

Additional context
Contexts are deprecated in HIP: https://rocm.docs.amd.com/projects/HIP/en/develop/doxygen/html/group___context.html

[loostrum]

Requires C++14, so blocked by #186