Bump MSRV to 1.81 #1244

Job	Run time
riscv64-zknh	1m 30s
riscv32-zknh	1m 30s
set-msrv / set-msrv	4s
minimal-versions / minimal-versions	35s
build (1.81.0, thumbv7em-none-eabi)	24s
build (1.81.0, wasm32-unknown-unknown)	22s
build (1.81.0, loongarch64-unknown-linux-gnu)	20s
build (stable, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	33s
build (stable, loongarch64-unknown-linux-gnu)	33s
cross (1.81.0, aarch64-unknown-linux-gnu, default)	38s
linux (i686-unknown-linux-gnu, 1.81.0, sudo apt update && sudo apt install gcc-multilib)	1m 3s
macos (1.81.0)	54s
linux (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 5s
macos (stable)	27s
cross (1.81.0, powerpc-unknown-linux-gnu, default)	52s
linux (x86_64-unknown-linux-gnu, 1.81.0)	42s
cross (stable, aarch64-unknown-linux-gnu, default)	31s
cross (stable, powerpc-unknown-linux-gnu, default)	32s
wasm (1.81.0, -C target-feature=-simd128)	1m 7s
linux (x86_64-unknown-linux-gnu, stable)	41s
wasm (stable, -C target-feature=-simd128)	50s
wasm (1.81.0, -C target-feature=+simd128)	35s
windows (1.81.0, x86_64-pc-windows-gnu)	1m 5s
windows (stable, x86_64-pc-windows-gnu)	1m 10s
wasm (stable, -C target-feature=+simd128)	35s
	18m 52s

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