[BUILD] CUDA 13 drops support for pre-Turing architectures

While trying to build OSL 1.15.5.0 with LLVM 22.1.7 and CUDA 13.3.0 I got the following build failure:

```
nvcc fatal : Unsupported gpu architecture 'sm_60'
```

when trying to build:

```
src/testshade/optix_grid_renderer.ptx
src/testrender/optix_raytracer.ptx
```

`CMakeLists.txt` has:

```
set (CUDA_TARGET_ARCH "sm_60" CACHE STRING "CUDA GPU architecture (e.g. sm_50)")
```

which calls for Pascal architecture. I updated my Conan recipe to set that to "sm_75" (Turing) when detecting CUDA 13 or newer, I don't know if OSL makes any claims about CUDA 13 compatibility yet, but it might be good to have `CMakeLists.txt` adjust the baseline GPU architecture based on the detected CUDA version?

Start in CMakeLists.txt at the CUDA_TARGET_ARCH definition and trace how the CUDA version and PTX targets are configured for src/testshade/optix_grid_renderer.ptx and src/testrender/optix_raytracer.ptx. Reproduce the failure with CUDA 13.3.0, then verify the configuration works for CUDA 13 while preserving the intended behavior for older CUDA versions.

Written by the indexing model from the issue text.