Bump `aead` to v0.6.0-rc.3; restore `getrandom` features #1208

Job	Run time
test (stable)	21s
test (1.85.0)	13s
build (1.85.0, armv7a-none-eabi)	18s
build (1.85.0, wasm32-unknown-unknown)	18s
build (stable, thumbv7em-none-eabi)	21s
build (stable, armv7a-none-eabi)	16s
build (1.85.0, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	18s
	2m 23s

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