base32ct+base64ct: unpin `proptest`; MSRV 1.65 #1798

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	54s
test (1.65.0)	51s
build (1.65.0, wasm32-unknown-unknown)	50s
test (stable)	29s
build (stable, thumbv7em-none-eabi)	20s
build (stable, wasm32-unknown-unknown)	15s
minimal-versions / minimal-versions	25s
	4m 4s

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