Bump MSRV to 1.81 #393

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 49s
clmul (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	2m 3s
build (1.65.0, wasm32-unknown-unknown)	1m 45s
clmul (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	29s
build (stable, thumbv7em-none-eabi)	7s
clmul (x86_64-unknown-linux-gnu, 1.65.0)	1m 43s
build (stable, wasm32-unknown-unknown)	7s
clmul (x86_64-unknown-linux-gnu, stable)	12s
armv8 (aarch64-unknown-linux-gnu, nightly)	1m 11s
autodetect (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	2m 3s
autodetect (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	25s
cross (aarch64-unknown-linux-gnu)	1m 11s
autodetect (x86_64-unknown-linux-gnu, 1.65.0)	1m 48s
cross (powerpc-unknown-linux-gnu)	1m 5s
autodetect (x86_64-unknown-linux-gnu, stable)	14s
soft (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	2m 1s
soft (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	27s
soft (x86_64-unknown-linux-gnu, 1.65.0)	1m 49s
soft (x86_64-unknown-linux-gnu, stable)	13s
	20m 42s

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