- Raise CUDA minimum from 9.0 to 11.4 (sm_70/Volta).
- Raise HIP minimum to GFX906 (Vega 20/MI50) / ROCm 5.6.
- Remove EIGEN_HAS_{CUDA,HIP,GPU}_FP16 guards — FP16 is always available
on sm_70+ and GFX906+.
- Remove obsolete __HIP_ARCH_HAS_* preprocessor branches.
- C++14 cleanup: remove pre-C++14 workarounds in GPU code.
- Fix NVCC warnings (deprecated register keyword, unreachable code,
tautological comparisons).
- Fix HIP test execution on gfx1151.
- Update CI configuration for new minimum versions.
This is in preparation of adding GPU tests to the CI, allowing
us to limit building/testing of GPU-specific tests for a given
GPU-capable runner.
GPU tests are tagged with the label "gpu". The new targets
```
make buildtests_gpu
make check_gpu
```
allow building and running only the gpu tests.
Ancient versions of CMake required else(), endif(), and similar block
termination commands to have arguments matching the command starting the block.
This is no longer the preferred style.
