switch to C++20 only #852

Job	Run time
clang-format	13s
linux_clang-14_cuda-11.2_debug_analysis	3m 48s
windows_cl-2022_debug_analysis	6m 20s
macos_xcode-14.2_debug_analysis	8m 11s
linux_gcc-12_debug_analysis	1m 29s
macos_xcode-14.2_release	8m 19s
macos_xcode-14.3.1_debug	8m 42s
windows_cl-2022_release	6m 45s
windows_cl-2022_debug	2h 24m 55s
linux_gcc-9_debug	1m 31s
linux_gcc-12_release_c++20	1m 35s
linux_gcc-13_debug	14m 41s
linux_clang-10_release	1m 39s
linux_clang-12_release	1m 55s
linux_clang-13_debug	1m 14s
linux_clang-16_debug_ubsan	1m 15s
linux_clang-16_debug_tsan	1m 12s
	3h 33m 44s

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