GPU technote updates for 1.32 (chapel-lang#23496)

Important changes:

- Reduces defensiveness in the language
- But at the same time, adds a `.. warning::` to emphasize unstability
of the interface
- Creates a "Tested Configurations" at the very end and moves that
information there
- Moves the "kernels are generated from loops" paragraph into
"Overview".
- Removes "CUDA as early as 7.x may work"
- Adds information about ROCm 5.x
- Adjusts the memory strategy default to be `array_on_device`
- Points to ROCm definition for correct `CHPL_GPU_ARCH` setting for AMD
gpus. The table there is much easier to consume:
https://rocm.docs.amd.com/en/latest/release/gpu_os_support.html#linux-supported-gpus
- Adds a short section "Chapel Tasks and GPU Execution"

[Reviewed by @stonea and @ShreyasKhandekar]