GPU: Raise CUDA/HIP minimum and remove legacy guards

- 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.

test/CMakeLists.txt

@@ -433,7 +433,7 @@ if(EIGEN_TEST_CUDA_NVC AND NOT CMAKE_CXX_COMPILER_ID MATCHES "NVHPC")
   message(WARNING "EIGEN_TEST_CUDA_NVC is set, but CMAKE_CXX_COMPILER does not appear to be nvc++.")
 endif()
 
-find_package(CUDA 9.0)
+find_package(CUDA 11.4)
 if(CUDA_FOUND AND EIGEN_TEST_CUDA)
   # Make sure to compile without the -pedantic, -Wundef, -Wnon-virtual-dtor
   # and -fno-check-new flags since they trigger thousands of compilation warnings
@@ -502,6 +502,9 @@ if (EIGEN_TEST_HIP)
   endif()
 
   find_package(HIP REQUIRED)
+  if (HIP_FOUND AND HIP_VERSION VERSION_LESS "5.6")
+    message(FATAL_ERROR "Eigen requires ROCm/HIP >= 5.6, found ${HIP_VERSION}")
+  endif()
   if (HIP_FOUND)
     execute_process(COMMAND ${HIP_PATH}/bin/hipconfig --platform OUTPUT_VARIABLE HIP_PLATFORM)