multiboot2: prepare v0.23.1 #1094

Job	Run time
build (msrv) / rust	39s
build (nightly) / rust	1m 13s
build (stable) / rust	35s
build no_std (stable) [w/o builder] / rust	29s
style (stable) / rust	31s
build no_std (stable) [Windows] / rust	2m 14s
build no_std (stable) / rust	18s
build no_std (msrv) / rust	28s
style (msrv) / rust	26s
build no_std (nightly) / rust	31s
style (nightly) / rust	20s
tests with miri (nightly) / rust	2m 52s
	10m 36s

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