Fix HIP build with ROCm 6, avoid deprecation warnings.

As of ROCm 6, the path /opt/rocm-6.0.0/hip no longer exists. (And it was previously throwing deprecation warnings as noted in other issues.)

Therefore, this MR adds a few changes:

• Both Make/CMake builds now look for ROCM_PATH instead of HIP_PATH. It turns out the latter was never actually set by e.g., OLCF ROCm modules anyway, so this removes one of our major impedance mismatches relative to how ROCm is deployed. Note: ROCM_PATH is set by default in most modules and does not need to be set by the user, but if for some reason you need to, set it to /opt/rocm-X.Y.Z and NOT to /opt/rocm-X.Y.Z/hip.
• We now include ONLY $ROCM_PATH/include and NOT $ROCM_PATH/hip/include, which fixes ROCm 6 and avoids deprecation warnings in ROCm 5.
• ROCm 4.x is no longer supported. ROCm 5 and up is now required.
• This change also has some miscellaneous fixes for our Frontier CI.

I have tested this in the CMake and Make builds with ROCm 5.1.0, 5.4.3, and 6.0.0.