Rename `cuda::experimental::managed_memory_resource` to `cuda::experimental::unified_memory_resource`

"Managed memory" is a terrible and unclear name for the CUDA Unified Memory feature, and it may be particularly confusing to C++ users, as there is an [existing, deprecated, and terrible thing called "Managed C++"](https://en.wikipedia.org/wiki/Managed_Extensions_for_C%2B%2B).

C++ people don't like things that are "managed"; people write C++ because they want control. This name makes Unified Memory sound like a scary black box with perf issues. Couple that with the fact that historically Unified Memory has had perf issues, and the name sounds the wrong sort of message.

We can't rename `cudaMallocManaged`, but we don't have to repeat the mistake. Please call it `cudax::experimental::unified_memory_resource` not `cudax::experimental::managed_memory_resource`.