🔥 Remove some pre-C++20 support #953

Job	Run time
sanitize (address, gcc)	5m 16s
sanitize (thread, clang)	5m 49s
sanitize (address, clang)	5m 55s
valgrind	5m 59s
sanitize (undefined, gcc)	5m 6s
sanitize (thread, gcc)	4m 53s
quality_checks_pass	1m 6s
build_and_test_24 (clang, 18, 23, libstdc++, Debug)	5m 46s
build_and_test_24 (clang, 20, 23, libstdc++, Debug)	5m 55s
build_and_test_24 (gcc, 13, 23, libstdc++, Debug)	4m 49s
build_and_test_24 (gcc, 12, 23, libstdc++, Debug)	5m 1s
mutate	5m 11s
build_and_test_24 (clang, 18, 23, libc++, Debug)	7m 33s
build_and_test_24 (clang, 22, 23, libstdc++, Debug)	6m 28s
build_and_test_24 (clang, 20, 23, libc++, Debug)	6m 56s
sanitize (undefined, clang)	6m 26s
build_and_test_24 (clang, 22, 23, libc++, Debug)	6m 30s
build_and_test_24 (clang, 19, 23, libc++, Debug)	6m 43s
build_and_test_24 (clang, 19, 23, libstdc++, Debug)	5m 40s
build_and_test_24 (clang, 21, 23, libc++, Debug)	6m 2s
build_and_test_24 (clang, 21, 23, libstdc++, Debug)	5m 48s
build_and_test_24 (gcc, 14, 23, libstdc++, Debug)	5m 12s
merge_ok	4s
	2h 4m 8s

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