base32ct+base64ct: unpin `proptest`; MSRV 1.65 #766

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	31s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	26s
build (stable, thumbv7em-none-eabi)	18s
test (1.75.0)	32s
build (stable, wasm32-unknown-unknown)	39s
test (stable)	33s
	2m 59s

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