Draft: [RFC] Remove support for using OpenCL as GPU backend

OpenCL has been deprecated for years and has not seen any of the new features add to the CUDA and SYCL backends.

It also restricts us when it comes to writing host code because we still need to be able to pass it to the OpenCL JIT compiler that only understands C99, meaning we can't use templated types as arguments.

Also remove clfft from the repository, as it was only used with OpenCL.

Closes #5079