base32ct+base64ct: unpin `proptest`; MSRV 1.65 #2319

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	50s
build (1.65.0, wasm32-unknown-unknown)	50s
test (1.65.0)	59s
build (stable, thumbv7em-none-eabi)	14s
test (stable)	18s
build (stable, wasm32-unknown-unknown)	19s
minimal-versions / minimal-versions	37s
	4m 7s

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