base32ct+base64ct: unpin `proptest`; MSRV 1.65 #1977

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	57s
build (1.65.0, wasm32-unknown-unknown)	56s
test (1.65.0)	48s
build (stable, thumbv7em-none-eabi)	12s
test (stable)	14s
build (stable, wasm32-unknown-unknown)	17s
minimal-versions / minimal-versions	27s
	3m 51s

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