chacha20: bump `rand_core` to v0.9 #639

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	18s
build (1.81.0, wasm32-unknown-unknown)	16s
test (1.81.0)	19s
build (stable, thumbv7em-none-eabi)	13s
test (stable)	12s
build (stable, wasm32-unknown-unknown)	14s
minimal-versions / minimal-versions	21s
	1m 53s

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