CUDA: Update default architecture resolver, docs, and CI to sm_60

- PacketMath.h: Update architecture comment to reflect sm_60+ support for __ldg and native FP16.
- EigenTesting.cmake: Default EIGEN_CUDA_COMPUTE_ARCH to 60 for CUDA < 13 so the default test target exercises the newly supported floor.
- CMakeLists.txt: Update cache help text to document that empty selects sm_60 (or sm_75 on CUDA 13+).
- ci/build.linux.gitlab-ci.yml: Add 60 to EIGEN_CI_CUDA_COMPUTE_ARCH for CUDA 11.8 CI builds to provide compile-time coverage for sm_60.
4 files changed