Bump MSRV to 1.81 #104

Job	Run time
minimal-versions	0s
set-msrv / set-msrv	3s
macos (1.81.0)	30s
linux (x86_64-unknown-linux-gnu, 1.81.0)	33s
macos (stable)	27s
build (1.81.0, thumbv7em-none-eabi)	24s
linux (x86_64-unknown-linux-gnu, stable)	31s
cross (1.81.0, aarch64-unknown-linux-gnu, default)	50s
build (1.81.0, wasm32-unknown-unknown)	24s
cross (1.81.0, powerpc-unknown-linux-gnu, default)	38s
windows (x86_64-pc-windows-gnu, 1.81.0)	1m 6s
build (1.81.0, loongarch64-unknown-linux-gnu)	28s
cross (stable, aarch64-unknown-linux-gnu, default)	33s
windows (x86_64-pc-windows-gnu, stable)	2m 6s
build (stable, thumbv7em-none-eabi)	18s
cross (stable, powerpc-unknown-linux-gnu, default)	31s
build (stable, wasm32-unknown-unknown)	30s
build (stable, loongarch64-unknown-linux-gnu)	34s
	10m 26s

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