base64ct: downgrade edition to 2021; MSRV 1.81 #1714

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	25s
build (1.81.0, wasm32-unknown-unknown)	22s
build (stable, thumbv7em-none-eabi)	26s
minimal-versions	0s
test (1.81.0)	41s
build (stable, wasm32-unknown-unknown)	16s
test (stable)	31s
	2m 41s

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