hmac: bump `digest` to v0.11.0-pre.8 (#156) #89

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	2m 44s
test (1.56.0)	2m 39s
build (1.56.0, wasm32-unknown-unknown)	2m 48s
build (stable, thumbv7em-none-eabi)	14s
test (stable)	8s
build (stable, wasm32-unknown-unknown)	9s
minimal-versions / minimal-versions	23s
	9m 5s

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