chacha20: bump `rand_core` to v0.9 #619

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	21s
test (1.81.0)	17s
build (1.81.0, wasm32-unknown-unknown)	15s
build (stable, thumbv7em-none-eabi)	9s
test (stable)	12s
build (stable, wasm32-unknown-unknown)	10s
minimal-versions / minimal-versions	24s
	1m 48s

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