schedule transition to C++20 as default #9560

Job	Run time
Windows (check, intel)	53m 7s
Windows (installer, intel)	22m 57s
Windows (installer, arm)	23m 27s
MacOS (macOS-15)	22m 58s
Windows (check, arm)	53m 42s
Linux (clang, debian:stable, arm)	32m 32s
Linux (clang, debian:testing, intel)	34m 42s
Linux (gcc, debian:testing, arm)	33m 7s
Linux (clang, debian:stable, intel)	34m 23s
Linux (gcc, debian:stable, arm)	33m 20s
Linux (gcc, debian:testing, intel)	34m 1s
MacOS (macOS-15-intel)	49m 51s
Linux (clang, debian:testing, arm)	32m 59s
Linux (gcc, debian:stable, intel)	33m 2s
	8h 14m 8s

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