Allow C++20 HIP builds (#4739)

This removes the hard-coded C++17 compiler flag when HIP is turned on,
so codes are able to use C++20 with HIP.

Author: BenWibking

Tools/CMake/AMReXParallelBackends.cmake

@@ -366,10 +366,8 @@ if (AMReX_HIP)
        # 
        target_compile_options(amrex_${D}d PUBLIC $<$<COMPILE_LANGUAGE:CXX>:-munsafe-fp-atomics>)
 
-       # ROCm 5.5: forgets to enforce C++17 (default seems lower)
-       # https://github.com/AMReX-Codes/amrex/issues/3337
-       #
-       target_compile_options(amrex_${D}d PUBLIC $<$<COMPILE_LANGUAGE:CXX>:-std=c++17>)
+       # Ensure ROCm builds enable at least C++17 without overriding higher standards
+       target_compile_features(amrex_${D}d PUBLIC cxx_std_17)
    endforeach()
 
    # Equivalently, relocatable-device-code (RDC) flags are needed for `extern`